Concerns I've heard with regard to VMs and VirusScan Enterprise, have been due to the System utilization code in the On-Demand Scanner (ODS).
The user interface says System utilization with a slider from 10% to 100%, which folks immediately assume reflects CPU usage but that is incorrect.
System utilization was based on CPU _and_ I/O throughput - I say "was" because that's how it used to work. Now, with any current patch level, it is based on thread priority.
Picture VirusScan running inside a VM, where the host server has multiple VMs.
One of those VMs has an on-demand scan task running, with System utilization set to 10% - meaning, the on-demand scan inside that VM will take a back seat to any other activity that other processes in the VM are engaged in. If there is no other activity occurring then the on-demand scan will use as much CPU as it can... until another process becomes active. The operating system's own algorithms for handling thread priority decide who gets CPU.
Lets assume that VM has no other active processes. Thus, the ODS will be using 100% CPU of that VM.
Do you see how this could affect other VMs on that host?
Those other VMs may not have an ODS running, they may not be doing anything at all - but if they are, their host server's CPU is being pegged by the VM running the ODS...
This is a use-case scenario hoped to be improved upon in future product releases.