Microsoft just released the Hyper-V security guide.
You can download it here, or read about it online here.
Chapter 1: Hardening Hyper-V This chapter provides prescriptive guidance
for hardening the Hyper-V role. It discusses several best practices for installing
and configuring Hyper-V on Windows Server 2008 server with a focus on security.
These best practices include measures for reducing the attack surface of a server
running Hyper-V and recommendations for properly configuring secure network
and storage devices.
Chapter 2: Delegating Virtual Machine Management This chapter discusses several
available methods for delegating virtual machine management so that virtual
machine administrators only have the minimum permissions they require.
It describes common delegation scenarios, and includes detailed steps to guide
you through using Authorization Manager (AzMan) and System Center Virtual Machine
Manager 2008 (VMM 2008) to separate virtual machine administrators from
virtualization host administrators.
Chapter 3: Protecting Virtual Machines This chapter provides prescriptive guidance
for securing virtual machine resources. It discusses best practices and includes
detailed steps for protecting virtual machines by using a combination of file
system permissions, encryption, and auditing. Also included are resources for
hardening and updating the operating system instances running within you
virtual machines.