Microsoft announced today with it will be publishing the Hypercall APIs through the Open Specification Promise when Windows Server Virtualization ships. In addition they posted a draft of these interfaces in the form of a Hypervisor top-level functional specification on Microsoft downloads here. This document gives you and view of the external interfaces of the first generation hypervisor from Microsoft and contains the following information:
This is really cool and gives you a detailed view of how to interface with the Microsoft hypervisor.
For more information, go to the Virtualization Product team blog entry here