Hyper-V™ is a key feature in Windows Server® 2008 and Windows Server 2008 R2 that you can use to consolidate different server roles onto a single physical computer.
Hyper-V is a hypervisor-based server technology that is optimized to provide virtualization of operating systems. Hyper-V supports a large variety of guest operating systems that can run concurrently on a single physical server. This guide specifically describes running domain controllers that use Windows Server 2003 or Windows Server 2008 on Hyper-V as guest operating systems. Hyper-V takes advantage of and requires a 64-bit architecture on the server’s operating system to provide improved performance and security. The guest operating systems can have either 32-bit architectures or 64-bit architectures.
With Hyper-V, you can install a Windows Server domain controller as a virtual machine alongside other application servers on a single physical Windows Server 2008 server. This can drastically reduce the number of physical computers in a datacenter, which in turn can reduce management and energy costs.