In computing, a HyperVisor (also: virtual machine monitor) is an OsVirtualization platform that allows multiple operating systems to run on a host computer at the same time. The term usually refers to an implementation using full virtualization. Hypervisors are currently classified in two types:
-
Type 1 hypervisor is software that runs directly on a given hardware platform (as an operating system control program). A "guest" operating system thus runs at the second level above the hardware. The classic type 1 hypervisor was CP/CMS, developed at IBM in the 1960s, ancestor of IBM's current z/VM. More recent examples are Xen, VMware's ESX Server, and Sun's Logical Domains Hypervisor (released in 2005).
-
Type 2 hypervisor (is software that runs within an operating system environment. A "guest" operating system thus runs at the third level above the hardware. Examples include VMware Server (formally GSX) and Workstation, as well as Microsoft's Virtual PC and Microsoft Virtual Server prodcuts.
The term hypervisor apparently originated in IBM's CP-370 reimplementation of CP-67 for the IbmSystemThreeSeventy, released in 1972 as VM/370. The term hypervisor call referred to the paravirtualization interface, by which a "guest" operating system could access services directly from the (higher-level) control program – analogous to making a "supervisor call" to the (same level) operating system. (The term 'supervisor' refers to the operating system kernel, which on IBM mainframes runs in supervisor state.)