1.3 What are Hypervisors?

If virtualization is defined as enabling multiple operating systems to run on a single host computer, then the essential component in the virtualization stack is the hypervisor. This hypervisor, also called Virtual Machine Monitor (VMM), creates a virtual platform on the host computer, on top of which multiple guest operating systems are executed and monitored. This way, multiple operating systems, which are either multiple instances of the same operating system, or different operating systems, can share the hardware resources offered by the host.

Hypervisors are commonly classified as one of these two types, as show in Table 1.2, “Hypervisor Types”.

Table 1.2 Hypervisor Types


Characteristics and Description

Type 1: native or bare metal

Native hypervisors are software systems that run directly on the host's hardware to control the hardware, and to monitor the guest operating systems. Consequently, the guest operating system runs on a separate level above the hypervisor. Examples of this classic implementation of virtual machine architecture are Oracle VM, Microsoft Hyper-V, VMware ESXi and Xen.

Type 2: hosted

Hosted hypervisors are designed to run within a traditional operating system. In other words, a hosted hypervisor adds a distinct software layer on top of the host operating system, and the guest operating system becomes a third software level above the hardware. A well-known example of a hosted hypervisor is Oracle VM VirtualBox. Others include VMware Server and Workstation, Microsoft Virtual PC, KVM, QEMU and Parallels.