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 is running one or more virtual machines is defined as a host machine. 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.
In their 1974 article "Formal Requirements for Virtualizable Third Generation Architectures" Gerald J. Popek and 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. A guest operating system runs as a process on the host. 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 z/VM). Modern equivalents include Oracle VM Server for SPARC, Oracle VM Server for x86, the Citrix XenServer, VMware ESX/ESXi and Microsoft Hyper-V 2008/2012.
Type-2 or hosted hypervisors
These hypervisors run on a conventional operating system just as other computer programs do. Type-2 hypervisors abstract guest operating systems from the host operating system. VMware Workstation, VMware Player, VirtualBox and QEMU are examples of type-2 hypervisors.
However, the distinction between these two types is not necessarily clear. Linux's Kernel-based Virtual Machine (KVM) and FreeBSD's bhyve are kernel modules that effectively convert the host operating system to a type-1 hypervisor.Nevertheless, since Linux distributions and FreeBSD are still general-purpose operating systems, with other applications competing for VM resources, KVM and bhyve can also be categorized as type-2 hypervisors.
Microsoft's Hyper-V is often run on top of Windows Server full installation. This would make it type-2 in this scenario.
In 2012, a US software development company called LynuxWorks proposed a type-0 (zero) hypervisor one with no kernel or operating system whatsoeverwhich might not be entirely possible.