Virtualization is a key component of the new architecture. Managing development, build and deployment environments on a variety of hardware and operating systems has always been extremely costly. We have a fairly low tolerance for inconsistencies in both environments, yet customization is critical to most developers and avoided at all costs in production. Virtualization is the technology that gives Genome isolation in both worlds. The development, build, and deployment environments can all be isolated and managed on virtual machines to enable different configurations and optimizations while still residing on the same machine. It also gives us the flexibility to modify our virtualization option as time progress (e.g. from Xen to KVM) but keep the core strategy consistent for the foreseable future.
RHEL 5 Documentation about Virtualization
Make sure you are comfortable with xen and how to create virtual machines, start them, stop them, and customize them.