Classification
Robert P. Goldberg classifies two types of hypervisor:
- Type 1 (or native, bare metal) hypervisors run directly on the host's hardware to control the hardware and to manage guest operating systems. A guest operating system thus runs on another level above the hypervisor.
- This model represents the classic implementation of virtual machine architectures; the original hypervisors were the test tool, SIMMON, and CP/CMS, both developed at IBM in the 1960s. CP/CMS was the ancestor of IBM's z/VM. Modern equivalents of this are Oracle VM Server for SPARC, the Citrix XenServer, KVM, VMware ESX/ESXi, and Microsoft Hyper-V hypervisor.
- Type 2 (or hosted) hypervisors run within a conventional operating system environment. With the hypervisor layer as a distinct second software level, guest operating systems run at the third level above the hardware. VMware Workstation and VirtualBox are examples of Type 2 hypervisors.
In other words, Type 1 hypervisor runs directly on the hardware; a Type 2 hypervisor runs on another operating system, such as FreeBSD, Linux or Windows.
Note: Microsoft Hyper-V (released in June 2008) exemplifies a type 1 product that can be mistaken for a type 2. Both the free stand-alone version and the version that is part of the commercial Windows Server 2008 product use a virtualized Windows Server 2008 parent partition to manage the Type 1 Hyper-V hypervisor. In both cases the Hyper-V hypervisor loads prior to the management operating system, and any virtual environments created run directly on the hypervisor, not via the management operating system.
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