Editing Hypervisor (section)

{{Short description|Piece of software or hardware that creates and runs virtual machines}}
{{Use mdy dates|date=May 2015}}

A '''hypervisor''', also known as a '''virtual machine monitor''' ('''VMM''') or '''virtualizer''', is a type of computer [[software]], [[firmware]] or [[computer hardware|hardware]] that creates and runs [[virtual machine]]s. A computer on which a hypervisor runs one or more virtual machines is called a ''host machine'', and each virtual machine is called a ''guest machine''. The hypervisor presents the guest operating systems with a [[platform virtualization|virtual operating platform]] and manages the execution of the guest operating systems. Unlike an [[emulator]], the guest executes most instructions on the native hardware.<ref name="goldberg1973">{{cite tech report |url=https://apps.dtic.mil/sti/pdfs/AD0772809.pdf |first=Robert P. |last=Goldberg  |title=Architectural Principles for Virtual Computer Systems |number=ESD-TR-73-105 |institution=Harvard University |year=1973}}</ref> Multiple instances of a variety of operating systems may share the virtualized hardware resources: for example, [[Linux]], [[Microsoft Windows|Windows]], and [[macOS]] instances can all run on a single physical [[x86]] machine. This contrasts with [[operating-system-level virtualization|operating-system–level virtualization]], where all instances (usually called ''containers'') must share a single kernel, though the guest operating systems can differ in [[user space]], such as different [[Linux distribution]]s with the same kernel.

The term ''hypervisor'' is a variant of ''supervisor'', a traditional term for the [[kernel (operating system)|kernel]] of an [[operating system]]: the hypervisor is the supervisor of the supervisors,<ref>{{cite book |title=Virtualization For Dummies |author=Bernard Golden |year=2011 |page=[https://books.google.com/books?id=2ppZkdmpSlgC&pg=PA54 54]}}</ref> with ''[[Wikt:hyper-|hyper-]]'' used as a stronger variant of ''[[Wikt:super-|super-]]''.{{efn|''super-'' is from Latin, meaning "above", while ''hyper-'' is from the [[cognate]] term in [[Ancient Greek]] ([[Wikt:ὑπέρ|ὑπέρ-]]), also meaning ''above'' or ''over''.}} The term dates to circa 1970;<ref>{{cite web |url=http://programmers.stackexchange.com/questions/196405/how-did-the-term-hypervisor-come-into-use |title=How did the term "hypervisor" come into use?}}</ref> IBM coined it for software that ran [[OS/360]] and the 7090 emulator concurrently on the [[IBM System/360 Model 65|360/65]]<ref>{{cite conference  | url = https://www.computer.org/csdl/proceedings/afips/1971/5077/00/50770163.pdf | title = System/370 integrated emulation under OS and DOS | author = Gary R. Allred | page = 164 | conference = 1971 [[Spring Joint Computer Conference]] | volume = 38 | doi = 10.1109/AFIPS.1971.58 | date = May 1971 | publisher = AFIPS Press | access-date = June 12, 2022 }}</ref> and later used it for the DIAG handler of CP-67. In the earlier [[CP/CMS]] (1967) system, the term ''Control Program'' was used instead.

Some literature, especially in [[microkernel]] contexts, makes a distinction between ''hypervisor'' and ''virtual machine monitor'' (VMM). There, both components form the overall ''virtualization stack'' of a certain system.  ''Hypervisor'' refers to  [[User_space_and_kernel_space|kernel-space]] functionality and VMM to [[User_space_and_kernel_space|user-space]] functionality. Specifically in these contexts, a ''hypervisor'' is a microkernel implementing virtualization infrastructure that must run in kernel-space for technical reasons, such as [[X86_virtualization|Intel VMX]]. Microkernels implementing virtualization mechanisms are also referred to as ''microhypervisor''.<ref>{{cite conference | last = Steinberg | first = Udo | last2 = Kauer
 | first2 = Bernhard | title = NOVA: A Microhypervisor-Based Secure Virtualization Architecture | book-title = Proceedings of the 2010 ACM European Conference on Computer Systems (EuroSys 2010) | date = 2010
 | location = Paris, France | url = https://hypervisor.org/eurosys2010.pdf | access-date = 2024-08-27
}}</ref><ref>{{cite web | url = https://github.com/cyberus-technology/hedron | title = Hedron Microkernel | website = GitHub | publisher = Cyberus Technology | access-date = 2024-08-27 }}</ref> Applying this terminology to [[Linux]], [[Kernel-based_Virtual_Machine|KVM]] is a ''hypervisor'' and [[QEMU]] or [[Cloud Hypervisor]] are VMMs utilizing KVM as hypervisor.<ref>{{cite web | url = https://github.com/cloud-hypervisor/cloud-hypervisor | title = Cloud Hypervisor | website = GitHub | publisher = Cloud Hypervisor Project | access-date = 2024-08-27 }}</ref>