The following table summarizes the differences between different Server OS's capable of Hyper-V.
http://www.microsoft.com/servers/hyper-v-server/how-to-get.mspx
The following table outlines which Hyper-V–enabled product would suit your needs:
Virtualization Needs
Microsoft Hyper-V Server 2008
Windows Server 2008 Standard
Windows Server 2008 Enterprise
Windows Server 2008 Datacenter
Server Consolidation
Test and Development
Mixed OS Virtualization (Linux and Windows)
Local Graphical User Interface
High Availability—Clustering
Quick Migration
Large Memory Support (Host OS) > 32 GB RAM
Support for > 4 Processors (Host OS)
Ability to Add Additional Server Roles
Guest Virtualization Rights Included in Host Server License
None—Each Windows Guest VM Requires a License
1 Physical + 1 VM*
1 Physical + 4 VMs*
1 Physical + Unlimited VMs (Free)
* Each additional Windows guest VM requires a license.
If you need to acquire and host new server licenses, Windows Server 2008 Standard, Enterprise, and Datacenter provide the best value.