A hypervisor or the virtual machine monitor (VMM) is a part of computer firmware, software or hardware, which runs and creates cybernetic machines.
When one or more than one virtual machines are run by the hypervisor on a computer, it is then known as the host machine. Guest Machine is the term given to all the virtual machines. The hypervisor in the guest OS (operating systems) along with the cybernetic operating platform, accomplishes the implementation of the guest operating systems. Several examples of a range of operating systems may divide the virtualized hardware resources amongst them.
In 1974, Robert P. Goldberg along with Gerald J. Popek classified hypervisor into two categories in the article ""Formal Requirements for Virtualizable Third Generation Architectures"":
Type 1: When the hypervisor works on the hardware of the host directly, to manage the hardware and control the guest or visitors operating system. Thus it is understood, by default, that the guest operating system is completely dependent on some other and higher level of hypervisor.
Type 2: Hypervisors function within the environment of conventional operating system. As hypervisor layer is the separate second software level, the visitors’ operating system or the guest operating system works in the level above the hardware.