View source for Software:Kernel-based Virtual Machine

{{short description|Virtualization module in the Linux kernel}}

{{Infobox software
| name                 = KVM
| logo                 = Kvmbanner-logo2 1.png
| logo size            = 160px
| screenshot           = QEMU 6.2 running NetBSD and OpenIndiana screenshot.png
| screenshot size      = 300px
| caption              = Screenshot of QEMU/KVM running [[Software:NetBSD|NetBSD]] and [[Software:OpenIndiana|OpenIndiana]] guests on an [[Software:Arch Linux|Arch Linux]] host.
| author               = [[Company:Qumranet|Qumranet]]
| developer            = The Linux Kernel community
| operating_system     = [[Unix-like]]
| programming_language = [[C (programming language)|C]]
| platform             = [[ARM architecture|ARM]], [[PowerPC]], ESA/390, [[IA-32]], [[X86-64|x86-64]]
| genre                = [[Hypervisor]]
| license              = [[Software:GNU General Public License|GNU GPL]] or [[Software:GNU Lesser General Public License|LGPL]]
| website              = {{URL|www.linux-kvm.org}}
}}

'''Kernel-based Virtual Machine''' ('''KVM''') is a free and open-source [[Virtualization|virtualization]] module in the [[Software:Linux kernel|Linux kernel]] that allows the [[Software:Kernel (operating system)|kernel]] to function as a [[Hypervisor|hypervisor]]. It was merged into the [[Software:Mainline Linux|mainline Linux kernel]] in version 2.6.20, which was released on February 5, 2007.<ref name="2620notes">{{cite web
 | url        = http://kernelnewbies.org/Linux_2_6_20#head-bca4fe7ffe454321118a470387c2be543ee51754
 | title      = Linux kernel 2.6.20, Section 2.2. Virtualization support through KVM
 | date       = 2007-02-05
 | access-date = 2014-06-16
 | website    = kernelnewbies.org}}</ref> KVM requires a processor with [[Hardware virtualization|hardware virtualization]] extensions, such as Intel VT or AMD-V.<ref>[http://www.linux-kvm.org/page/FAQ#What_do_I_need_to_use_KVM.3F KVM FAQ: What do I need to use KVM?]</ref> KVM has also been ported to other operating systems such as [[Software:FreeBSD|FreeBSD]]<ref>{{cite web|url=http://www.freebsd.org/news/status/report-2007-07-2007-10.html#Porting-Linux-KVM-to-FreeBSD|title=FreeBSD Quarterly Status Report: Porting Linux KVM to FreeBSD}}</ref> and [[Software:Illumos|illumos]]<ref>{{cite web|url=http://dtrace.org/blogs/bmc/2011/08/15/kvm-on-illumos/|title=KVM on illumos}}</ref> in the form of loadable kernel modules.

KVM was originally designed for [[X86|x86]] processors but has since been [[Porting|ported]] to ESA/390,<ref>{{Cite web |url=http://article.gmane.org/gmane.comp.emulators.kvm.devel/2570 |title=Gmane - Mail To News And Back Again<!-- Bot generated title --> |access-date=2007-05-07 |archive-url=https://web.archive.org/web/20070929124348/http://article.gmane.org/gmane.comp.emulators.kvm.devel/2570 |archive-date=2007-09-29 |url-status=dead }}</ref> [[PowerPC]],<ref>[http://news.gmane.org/gmane.comp.emulators.kvm.devel/2595 Gmane Loom<!-- Bot generated title -->] {{webarchive|url=https://web.archive.org/web/20070929103042/http://news.gmane.org/gmane.comp.emulators.kvm.devel/2595 |date=2007-09-29 }}</ref> [[Engineering:IA-64|IA-64]], and [[ARM architecture|ARM]].<ref>{{Cite web |url=https://columbia.github.com/linux-kvm-arm |title=KVM/ARM Open Source Project |access-date=2017-11-01 |archive-url=https://web.archive.org/web/20130310052146/http://columbia.github.com/linux-kvm-arm/ |archive-date=2013-03-10 |url-status=dead }}</ref> The IA-64 port was removed in 2014.<ref>{{Cite web|url=https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=003f7de6258900e17f6206e8e417d76c75ca549f | title=kernel/git/torvalds/linux.git: KVM: ia64: remove}}</ref>

KVM supports [[Hardware-assisted virtualization|hardware-assisted virtualization]] for a wide variety of guest operating systems including BSD, [[Software:Solaris (operating system)|Solaris]], [[Software:Microsoft Windows|Windows]], [[Software:Haiku (operating system)|Haiku]], [[Software:ReactOS|ReactOS]], [[Software:Plan 9 from Bell Labs|Plan 9]], [[Software:AROS Research Operating System|AROS]], [[Software:MacOS|macOS]], and even other Linux systems.<ref>{{cite web | url = http://www.linux-kvm.org/page/Guest_Support_Status | title = KVM wiki: Guest support status | access-date =2007-05-27 }}</ref><ref>{{cite web|url=http://www.contrib.andrew.cmu.edu/~somlo/OSXKVM/|title=Running Mac OS X as a QEMU/KVM Guest|access-date=2014-08-20}}</ref> In addition, Android 2.2, GNU/Hurd<ref>{{cite web|url=https://www.gnu.org/software/hurd/hurd/status.html |title=status |publisher=Gnu.org |access-date=2014-02-12}}</ref> ([[Software:Debian|Debian]] K16), [[Software:MINIX 3|Minix]] 3.1.2a, Solaris 10 U3 and [[Software:Darwin (operating system)|Darwin]] 8.0.1, together with other operating systems and some newer versions of these listed, are known to work with certain limitations.<ref>{{cite web|url=http://www.linux-kvm.org/page/Guest_Support_Status |title=Guest Support Status - KVM |publisher=Linux-kvm.org |access-date=2014-02-12}}</ref>

Additionally, KVM provides [[Software:Paravirtualization|paravirtualization]] support for Linux, [[Organization:OpenBSD|OpenBSD]],<ref>{{cite web |url=https://man.openbsd.org/virtio.4|title=OpenBSD man page virtio(4) |access-date=2018-02-04}}</ref> FreeBSD,<ref>{{cite web |url=http://people.freebsd.org/~kuriyama/virtio/|title=virtio binary packages for FreeBSD|access-date=2012-10-29}}</ref> [[Software:NetBSD|NetBSD]],<ref>{{cite web |url=http://netbsd.gw.com/cgi-bin/man-cgi?virtio++NetBSD-current |title=NetBSD man page virtio(4) |access-date=2013-07-15 |archive-date=2019-11-13 |archive-url=https://web.archive.org/web/20191113035952/https://netbsd.gw.com/cgi-bin/man-cgi?virtio++NetBSD-current |url-status=dead }}</ref> Plan 9<ref>{{cite web |url=https://code.google.com/p/plan9front/wiki/qemu |title=plan9front |access-date=2013-02-11}}</ref> and Windows guests using the VirtIO [[API]].<ref>{{cite web |url=https://lwn.net/Articles/239238/ |title=An API for virtual I/O: virtio |date=2007-07-11 |access-date=2014-04-16 |publisher=LWN.net}}</ref> This includes a paravirtual Ethernet card, disk I/O controller,<ref>{{cite web |url=http://linux-iscsi.org/wiki/vHost |title=SCSI target for KVM wiki |publisher=linux-iscsi.org |date=2012-08-07 |access-date=2012-08-12 |archive-date=2020-06-05 |archive-url=https://web.archive.org/web/20200605120413/http://linux-iscsi.org/wiki/VHost |url-status=dead }}</ref> balloon driver, and a VGA graphics interface using SPICE or [[Company:VMware|VMware]] drivers.

==History==
Avi Kivity began the development of KVM in mid-2006 at [[Company:Qumranet|Qumranet]], a technology [[Finance:Startup company|startup company]]<ref>[http://kerneltrap.org/node/8088 Interview: Avi Kivity] {{webarchive|url=https://web.archive.org/web/20070426033902/http://kerneltrap.org/node/8088 |date=2007-04-26 }} on KernelTrap</ref> that was acquired by [[Company:Red Hat|Red Hat]] in 2008.<ref>{{cite web
 | url        = http://www.redhat.com/en/about/press-releases/qumranet
 | title      = Red Hat Advances Virtualization Leadership with Qumranet, Inc. Acquisition
 | date       = 4 September 2008
 | access-date = 16 June 2015
 | publisher  = [[Company:Red Hat|Red Hat]]}}</ref>

KVM surfaced in October 2006<ref>{{Cite web|url=https://www.itworldcanada.com/article/kvm-15-equipped-with-live-migration/7901|title=KVM 15 equipped with live migration &#124; IT World Canada News|date=7 March 2007}}</ref> and was merged into the Linux kernel mainline in kernel version 2.6.20, which was released on 5 February 2007.<ref name="2620notes" />

KVM is maintained by Paolo Bonzini.<ref>{{cite web
 | url          = http://www.linux.com/news/featured-blogs/200-libby-clark/821899-git-success-stories-and-tips-from-kvm-maintainer-paolo-bonzini
 | title        = Git Success Stories and Tips from KVM Maintainer Paolo Bonzini
 | date         = 7 April 2015
 | access-date   = 17 June 2015
 | author       = Libby Clark
 | publisher    = Linux.com
 | archive-url  = https://web.archive.org/web/20160315095510/http://www.linux.com/news/featured-blogs/200-libby-clark/821899-git-success-stories-and-tips-from-kvm-maintainer-paolo-bonzini
 | archive-date = 15 March 2016
 | url-status   = dead
 | df           = dmy-all}}</ref>

== Internals ==
[[File:Kernel-based Virtual Machine.svg|thumb|right|upright=1.8|A high-level overview of the KVM/QEMU virtualization environment<ref>{{cite web
 | url        = http://www-01.ibm.com/support/knowledgecenter/api/content/nl/en-us/linuxonibm/liaav/LPCKVMSSPV2.1.pdf
 | title      = KVM/QEMU Storage Stack Performance Discussion
 | year       = 2010
 | access-date = January 3, 2015
 | author1    = Khoa Huynh
 | author2    = Stefan Hajnoczi
 | publisher  = Linux Plumbers Conference
 | website    = IBM
 }}{{dead link
 | date          = January 2018
 | bot           = InternetArchiveBot
 | fix-attempted = yes}}</ref>{{rp|3}}]]

KVM provides device abstraction but no processor emulation. It exposes the {{mono|/dev/kvm}} interface, which a user mode host can then use to:
* Set up the guest VM's address space. The host must also supply a firmware image (usually a custom BIOS when emulating PCs) that the guest can use to bootstrap into its main OS.
* Feed the guest simulated I/O.
* Map the guest's video display back onto the system host.

Originally, a forked version of [[Software:QEMU|QEMU]] was provided to launch guests and deal with hardware emulation that isn't handled by the kernel. That support was eventually merged into the upstream project. There are now numerous Virtual Machine Monitors (VMMs) which can utilise the KVM interface including kvmtool, crosvm and [[Software:Firecracker|Firecracker]] and numerous specialised VMMs build with frameworks such as rust-vmm.

Internally, KVM uses [[Software:SeaBIOS|SeaBIOS]] as an open source implementation of a 16-bit x86 [[BIOS]].<ref>{{cite web|url=http://www.seabios.org/SeaBIOS |title=SeaBIOS |publisher=seabios.org |date=2013-12-21 |access-date=2014-06-16}}</ref>

==Features==
KVM has had support for [[Hot swapping|hot swappable vCPUs]],<ref>{{Cite web|url=https://access.redhat.com/articles/1339413|title=Hot Plugging Virtual CPUs with Red Hat Enterprise Virtualization Manager}}</ref> dynamic memory management,<ref>{{Cite web|url=https://www.linux-kvm.org/page/FAQ#Is_dynamic_memory_management_for_guests_supported.3F|title=Faq - KVM}}</ref> and Live Migration since February 2007.<ref>{{Cite web|url=https://lwn.net/Articles/223754/|title = KVM-15 release &#91;LWN.net&#93;}}</ref><ref>{{Cite web|url=https://www.linux-kvm.org/page/Migration|title = Migration - KVM}}</ref> It also reduces the impact that memory write-intensive workloads have on the migration process.<ref>{{Cite web|url=https://www.berrange.com/posts/2016/05/12/analysis-of-techniques-for-ensuring-migration-completion-with-kvm/|title=Daniel P. Berrangé » Blog Archive » Analysis of techniques for ensuring migration completion with KVM}}</ref>

==Emulated hardware==

KVM itself emulates very little hardware, instead deferring to a higher level client application such as [[Software:QEMU|QEMU]], crosvm, or [[Software:Firecracker|Firecracker]] for device emulation.

KVM provides the following emulated devices:

* Virtual CPU and memory<ref>{{cite web | url=https://www.kernel.org/doc/html/latest/virt/kvm/api.html#kvm-create-irqchip | title=The Definitive KVM (Kernel-based Virtual Machine) API Documentation — the Linux Kernel documentation }}</ref>
* VirtIO

==Graphical management tools==
[[File:Libvirt support.svg|thumb|300px|libvirt supports KVM]]

* [[Software:Kimchi|Kimchi]]{{snd}} web-based virtualization management tool for KVM
* [[Software:Virtual Machine Manager|Virtual Machine Manager]]{{snd}} supports creating, editing, starting, and stopping KVM-based virtual machines, as well as live or cold drag-and-drop migration of VMs between hosts.
* [[Software:Proxmox Virtual Environment|Proxmox Virtual Environment]]{{snd}} an open-source virtualization management package including KVM and [[Software:LXC|LXC]]. It has a bare-metal installer, a web-based remote management GUI, a HA cluster stack, unified storage, flexible network, and optional commercial support.
* [[Software:OpenQRM|OpenQRM]]{{snd}} management platform for managing heterogeneous data center infrastructures
* [[Software:GNOME Boxes|GNOME Boxes]]{{snd}} Gnome interface for managing libvirt guests on Linux
* [[Software:OVirt|oVirt]]{{snd}} open-source virtualization management tool for KVM built on top of [[Software:Libvirt|libvirt]]

==Licensing==
The kernel-mode component of KVM is a part of [[Software:Linux kernel|Linux kernel]], itself licensed under [[Software:GNU General Public License#Version 2|GNU General Public License, version 2]].<ref name="Linux_Licensing">{{Cite web|url=https://www.kernel.org/doc/html/latest/process/license-rules.html#kernel-licensing|title=Linux kernel licensing rules — The Linux Kernel documentation|website=www.kernel.org|access-date=2020-01-06|archive-date=7 March 2020|archive-url=https://web.archive.org/web/20200307065451/https://www.kernel.org/doc/html/latest/process/license-rules.html#kernel-licensing|url-status=live}}</ref>

==See also==

{{Div col|colwidth=20em}}
* CloudStack
* [[Software:Comparison of platform virtualization software|Comparison of platform virtualization software]]
* [[Kernel same-page merging]] (KSM)
* [[Software:Lguest|Lguest]]
* [[Software:Libguestfs|libguestfs]]
* [[Organization:Open Virtualization Alliance|Open Virtualization Alliance]]
* [[Software:OpenNebula|OpenNebula]]
* [[Software:OpenStack|OpenStack]]
* [[Software:Red Hat Virtualization|Red Hat Virtualization]]
* [[Software:Vx32|Vx32]]
* [[Software:Xen|Xen]]
{{div col end}}

==References==
{{Reflist|30em}}

==Bibliography==
* {{cite web|access-date=2017-02-10|title=Ten years of KVM|url=https://lwn.net/Articles/705160/|website=lwn.net|author=Amit Shah|date=2016-11-02}}

==External links==
* [http://public.dhe.ibm.com/software/dw/linux390/perf/ZSW03346USEN.pdf Best practices for the Kernel-based Virtual Machine], IBM, second edition, April 2012
* [https://web.archive.org/web/20141021055601/http://www.linux-kvm.org/wiki/images/f/f9/2012-forum-virtio-blk-performance-improvement.pdf Virtio-blk Performance Improvement], KVM Forum 2012, November 8, 2012, by Asias He
* [https://web.archive.org/web/20101128114720/http://qemu-buch.de/e/Content Wikibook QEMU & KVM]
* [https://chromium.googlesource.com/chromiumos/platform/crosvm/ crosvm - Chrome OS virtual machine monitor]
* [https://firecracker-microvm.github.io/ Firecracker VMM for KVM]


{{Virtualization products}}
{{Linux kernel}}


{{DEFAULTSORT:Kernel-based Virtual Machine}}
[[Category:Free emulation software]]
[[Category:Free software programmed in C]]
[[Category:Free virtualization software]]
[[Category:Linux kernel features]]
[[Category:Virtualization software]]

{{Sourceattribution|Kernel-based Virtual Machine}}