You don't so much "license" Redhat as "subscribe" to it. Your subscriptions require you to register the hosts in RedHat Network (RHN).
This article explains how to register virtual hosts on RHN:
It doesn't specifically talk about Hyper V but it seems the same would apply.
The link below talks about MS-Redhat cooperation on Hyper-V and RHEL:
Of course RHEL provides XEN so you don't have to use Hyper-V if all your guests are going to be RHEL. Why pay for a Windows license if you don't need to do so?