To install Hyper-V on a Server Core OS, execute the following two commands:

• BCDEdit /set hypervisorlaunchtype auto
• start /w OCSetup Microsoft-Hyper-V

The documentation for Hyper-V does not mention the BCDEdit step.  OCSetup will automatically perform the BCDEdit step normally, but it will only perform that operation after the first reboot following the Hyper-V installation.  If you don't perform the first step manually, it will take a second reboot in order to get the Hypervisor to load.  Thanks to MSDN's Mike Kolitz and his blog Virtual Varia for pointing this out.

Note: You may have to start the Hyper-V installation prior to running BCDEdit, as BCDEdit will throw an error if it realizes there is no Hypervisor to enable.  In such a case, fire up Task Manager and open a second command prompt, so you'll have somewhere to fire off the BCDEdit command prior to OCSetup's required reboot.