Compare Hyper-V versions and features

To make the right decision it’s very important to understand the information below.

Customers should use Windows Server 2008 R2 to get application failover capabilities or flexible and cost-effective virtualization rights (one free virtual instance for Standard Edition, four free virtual instances for Enterprise Editions, and unlimited virtual instances for Datacenter Edition with the purchase of a license of Windows Server 2008), and flexible and cost-effective virtualization rights.

You can use Hyper-V Server 2008 R2 for FREE (no license) it’s a kind of Windows 2008 R2 core version that can only be used as Hyper-V node.
Advantages: Free to use, less management, less patch management and a better performance.
Disadvantages: You need a license for each virtual Windows guest system.

The following table showcases specific scenarios where Hyper-V Server 2008 R2 should be used:

See the following Microsoft website for more information: 
http://www.microsoft.com/hyper-v-server/en/us/default.aspx