The Hypervisor

The hypervisor is what makes virtualization possible. The hypervisor is the component that translates the virtual machines into the underlying hardware. There are two types of hypervisors: The type 1 hypervisor runs directly on the host hardware; the type 2 or hosted hypervisor runs in software. In addition, some proprietary hardware has a hypervisor built into the hardware. The hypervisors that we are concerned with are the type 1 and type 2 hypervisors.

Type 1 Hypervisor

The type 1 (or embedded) hypervisor is a layer that runs directly on the host hardware, interfacing with the CPU, memory, and devices. Oracle VM and VMware ESX Server both use the type 1 hypervisor. The hypervisor treats the host OS in much the same way as a guest OS. The host OS is referred to as Domain 0 or dom0 and guests are referred to as Domain U or domU, as shown in Figure 1-1. Here you can see that all virtual machines must go through the hypervisor to get to the hardware. The dom0 domain is a virtual system just like the domU virtual machines (but has more capabilities as discussed later). Currently, the type 1 hypervisor is considered the most efficient and is the most recommended hypervisor.