Hypervisor
lastmod 2024-05-16created 2015-10-159 edits
A hypervisor is a logical platform for running multiple operating systems simultaneously on a host computer. It is also called a virtual machine monitor (VMM for short).
Categories
- Virtualization
- Compatibility layer
- Hypervisor
- Virtual Machine
- Paravirtualization
- Full virtualization
- Emulator
- Cross-platform virtualization
- Computing platform
- Toolchain
Classification
Hypervisors are generally divided into two types.
Hypervisor_-_Classification.png
Type 1 (native or bare-metal)
Just as an operating system controls programs, the hypervisor runs directly on the hardware, and guest operating systems run at the second level above the hardware. This kind of hypervisor started with CP/CMS, developed by IBM in the 1960s, and continued into IBM's z/VM. Recent examples include Xen, Citrix's XenServer, VMware's ESX Server, the L4 microkernel, TRANGO, IBM's POWER hypervisor (PR/SM), Microsoft's Hyper-V, Parallels Server, and Sun's Logical Domains Hypervisor. Hypervisors are also sometimes embedded in platform firmware, like Hitachi's Virtage hypervisor, and KVM puts a complete Linux kernel inside the hypervisor, which is also Type 1.
Type 2 (hosted)
The hypervisor runs on a host operating system like an ordinary program, and the guest operating system running inside the VM runs at the third level above the hardware. Representative VMs include VMware Server, VMware Workstation, VMware Fusion, QEMU, Microsoft's Virtual PC and Virtual Server, Oracle (SUN)'s Virtual Box, and SWsoft's Parallels Workstation and Parallels Desktop.
The term hypervisor originated from CP-370, a rewrite of CP-67 for System/370, released in 1972 as VM/370. A hypervisor call, i.e. a hypercall, refers to a paravirtualization interface through which a guest operating system can access services directly from the (higher-level) control program. - It is similar to requesting a supervisor call from an operating system (at the same level). (A supervisor refers to the operating system kernel that runs in supervisor state on IBM mainframes.)