Here is some preliminary documentation on planning for backing up Hyper-V VMs. It will eventually be made part of our Planning and Deployment guide on TechNet.
Planning for Backup: When you plan a backup and recovery strategy for a virtualized server environment, there are several factors to consider. You must consider the different types of backups you can make, the state of the virtual machine, and the type of storage being used by the virtual machines. This topic discusses the advantages, disadvantages, and considerations for these factors.
Understanding the backup options and considerations: The backup integration service and the Hyper-V Volume Shadow Copy Service (VSS) Writer provide the mechanism for backing virtual machines as well as system-wide settings that apply to Hyper-V. There are two basic methods you can use to perform a backup.[…]