What is "hypervisor"

n. (context computing English) A software and/or firmware system that provides a virtual machine and allows it to operate directly on underlying hardware (instead of via emulation), but within specified constraints.

A hypervisor or virtual machine monitor (VMM) is a piece 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. Multiple instances of a variety of operating systems may share the virtualized hardware resources: for example, Linux, Windows, and OS X instances can all run on a single physical x86 machine. This contrasts with operating-system-level virtualization, where all instances 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 supervisor, with hyper- used as a stronger variant of super-. The term dates to circa 1970; in the earlier CP/CMS (1967) system the term Control Program was used instead.