If the only thing you are running on the virtualization server is virtual machines. why do you need to run anti-virus or other software that you normally run on a Windows Server 2008 R2 server?
Review this short video showing what happens when you enable the Hyper-V role (from Microsoft Showcase), and change your software installation so that all the VMs get the correct software installed as required by your IT policy. Review the best practice recommendations at Planning for Hyper-V Security (TechNet Library) and evaluate if they fit your environment. If your IT policy requires AV in the management operations system (on the host - as opposed to on each VM), see the list of Hyper-V AV exclusions in the primary AV exclusion KB for Enterprise customers (KB822158).