Client Hyper-V is the virtualization technology built into Windows 8. Client Hyper-V is the same virtualization technology previously available only in Windows Server. A similar functionality in Windows 7 is called Windows XP Mode.
Client Hyper-V enables you to run more than one 32-bit or 64-bit x86 operating system at the same time on the same host computer. But instead of working directly with the computer’s hardware, the operating systems run inside a virtual machine (VM). Hyper-V enables developers and IT professionals to easily maintain multiple test environments and provides a simple mechanism to quickly switch between these environments.