One of the most common misconceptions in the realm of Microsoft Virtualization products is the difference between the Hyper-V role and Hyper-V Server 2008 R2.
One of the biggest sources of confusion when dealing with Microsoft virtualization technologies is mistaking they Hyper-V role for the same thing as Hyper-V Server 2008 R2.
So what is the difference?
Microsoft Hyper-V Server 2008 R2 is actually a free stripped down version of the Windows Server 2008 operating system designed to be a simplified virtualization solution for consolidating servers, development, and testing environments.
The Hyper-V role is a feature of the Standard, Enterprise and Datacenter edition of Windows Server 2008 R2 that allows for a much more robust virtualization experience. When using the Hyper-V role with Windows Server 2008 R2 Standard, Enterprise, or Datacenter you gain several additional features including a local GUI, high availability (clustering) services, support for large amounts of memory, support for a large amount of processors, and the ability to add additional server roles. Along with this, these versions of Windows Server 2008 R2 include licensing for additional virtual machines which help to decrease cost when deploying new operating systems.