A NetVault Backup Server can be hosted on a virtual machine, as indicated in the NetVault Backup Compatibility Guide’s "Supported Virtual Machine Platforms" section. For details, please refer to the compatibility guide, which is available for download at the following link:
To be within the supported configuration you must stick to a very specific set of parameters listed on page 6 of that same section of the guide. The main limitation of hosting your NVBU Server on a virtual machine is the following:
--> "Storage devices directly attached to an ESX Server are not supported. This includes both direct-attached SCSI devices as well as devices on Fibre Channel that are exposed to the virtual machine through a RAW device mapping. "
This would mean that any physical storage devices must be attached to a SmartClient, which *is* running on physical box. This would also require the purchase of a SmartClient license.