Hyper-V, Microsoft’s newest entry into the virtualization arena, is built on the Windows 2008 Server platform. Hyper-V is installed as an add-on role within the 64-bit version of Windows 2008 Enterprise and Database Center editions. It allows for the creation of multiple child partitions (VM's) to run simultaneously on a single, physical server.  Each child partition is capable of running a different operating system.  

Child partitions are given the illusion that they are exclusively sit on-top-of the physical hardware. This is accomplished by taking advantage of hardware assisted virtualization. This hardware assisted virtualization allows the child partition to access the CPU and memory without having to first go through the parent partition. (The parent partition is the operating system installed directly onto the physical server). To access the CPU or memory both parent and child go through the same 300Kb hypervisor. Placing the hardware closer to the child partition is the secret to Hyper-V performance gains.

Only certain operating systems are able to take full advantage of the hypervisor.  These operating systems are referred to as Enlightened OS’s.   An example of difference between enlightened and non-enlightened operating systems would be how they access disk resources. An enlightened operating system can have physical disks directly “passed-through” from the parent partition without having to involve the rest of its disk model. Alternatively, an operating system without the enlightenments, attempting to access disk resources must pass through the full disk model of the parent partition. This additional (emulation) overhead substantially reduces throughput.

To date the following operating systems are enlightened:

Windows Server 2008
Windows Server 2003 SP2
Windows Server 2000 SP4
Windows HPC Server 2008
SUSE Linux Enterprise Server 10 SP1/SP2
Windows Vista SP1 (except home editions)
Windows XP SP2/SP3