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:

• Introduction
• Basic Data Types, Concepts and Notation
• Feature and Interface Discovery
• Hypercall Interface
• Partition Management
• Physical Hardware Management
• Resource Management
• Guest Physical Address Spaces
• Intercepts
• Virtual Processor Management
• Virtual Processor Execution
• Virtual MMU and Caching
• Virtual Interrupt Control
• Inter-Partition Communication
• Timers
• Message Formats
• Partition Save and Restore
• Scheduler
• Event Logging
• Guest Debugging Support
• Statistics
• Booting
• Appendix

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