Chapter 1: Introduction to Virtualization 17 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.