Read this blog post: http://blogs.technet.com/b/jeff_stokes/archive/2010/05/19/anti-virus-exclusions-and-you.aspx
Read also the following links:
On any Hyper-V host you will find a couple of core processes that is crucial to host and VM performance. Prevent the following processes from AV scans by excluding the following as part of you Hyper-V AV policy.
You also want to exclude the root directories where VM configurations and Virtual Hard Disks are stored. Exclude the following directories.
C:\Users\Public\Documents\Hyper-V\Virtual Hard Disks
Custom VM configuration, Virtual Hard Disk and Snapshot directories
Next, you want to create AV exclusions for the following file extensions.
Finally, if you are using Hyper-V R2’s Live Migration feature with Cluster Shared Volumes, then you will need to exclude the CSV path and any sub-directories. The CSV path is as follows.
Failure to create this exclusion on hosts using CSV, can not only result in poor performance, but can also result in a missing or corrupt VM configuration