Hypervisor

A hypervisor (also known as a virtual machine monitor, VMM, or virtualizer) 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, 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 hyper- used as a stronger variant of super-. The term dates to circa 1970; IBM coined it for 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. In his 1973 thesis, "Architectural Principles for Virtual Computer Systems," Robert P. Goldberg classified two types of hypervisor: Type-1, native or bare-metal hypervisors These hypervisors run directly on the host's hardware to control the hardware and to manage guest operating systems. For this reason, they are sometimes called bare-metal hypervisors. The first hypervisors, which IBM developed in the 1960s, were native hypervisors. These included the test software SIMMON and the CP/CMS operating system, the predecessor of IBM's VM family of virtual machine operating systems. Type-2 or hosted hypervisors These hypervisors run on a conventional operating system (OS) just as other computer programs do.