Beautiful Architecture

The Changing Shape of Xen

The traditional representation of a Xen-based system shows several virtual machines (known in Xen as domains) sitting on top of the hypervisor, which itself sits directly on the hardware (Figure 7-4). When it boots up, the hypervisor launches a special domain, known as domain zero. Domain zero has special privileges that allow it to manage the rest of the system, and it is analogous to a root or administrator process in a regular operating system. Figure 7-4 shows a typical Xen-based system, with domain zero and several guest domains (known as DomUs in Xen jargon) running on top of the Xen hypervisor.

Figure 7-4. Xen system architecture

Hosted Virtualization

Xen is an example of native virtualization (also known as Type 1 virtualization). The alternative approach is to run a hypervisor on top of a host operating system. In this case, each virtual machine effectively becomes a process in the host operating system. The host operating system is responsible for the management functions that domain zero performs on Xen. The hosted hypervisor and management software are like a regular application, which sits on top of (and might plug into) a commodity operating system; see Figure 7-5.

Hosted hypervisors are commonly used in the “workstation” versions of other virtualization products, such as VMWare Workstation, Parallels Workstation, and Microsoft Virtual PC. The main ...