The HV has RAM in use, but there are no VMs on it.
There are a couple of services that are always running on a HV, so they will use up RAM. If you are running Integrated Storage, then you will have a storage controller running on your HV. This controller will be using a portion of the RAM, generally about 2 GB or more, depending on your settings.
If you are running a XEN HV, then the dom0 device will be using RAM.
The OnApp system also has a checking system that is constantly running, using snmpd.