Topic summary
Hypervisor
A hypervisor, also known as a virtual machine monitor (VMM), is a type of computer software, firmware, or hardware that creates and runs virtual machines. A computer on which a hypervisor runs one or more virtual machines is called a host machine or virtualization server, and each virtual machine is called a guest machine. The hypervisor presents the guest operating systems with a virtual operating platform and manages the execution of the guest operating systems. Unlike an emulator, the guest executes most instructions on the native hardware. Multiple instances of a variety of operating systems may share the virtualized hardware resources: for example, Linux, Windows, and macOS instances can all run on a single physical x86 machine. This contrasts with 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 distributions with the same kernel.
The term hypervisor is a variant of supervisor, a traditional term for the kernel of an operating system: the hypervisor is the supervisor of the supervisors, with used as a stronger variant of . The term dates to circa 1970; IBM coined it for software that ran OS/360 and the 7090 emulator concurrently on the 360/65 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 kernel-space functionality and VMM to 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 Intel VMX. Microkernels implementing virtualization mechanisms are also referred to as microhypervisor. Applying this terminology to Linux, KVM is a hypervisor and QEMU or Cloud Hypervisor are VMMs utilizing KVM as hypervisor.