Let me clarify for you what 'Virtual Drive'/'Virtual HardDrive'/'Virtual HardDisk'/'Virtual HDD' means. Virtual Drive refers to the .hds/hdd file residing inside the .pvm package of a virtual machine, it's 'virtual' as opposed to 'real/physical' because the whole 'hard disk/partition' is a file. In a similar way an .iso or .dmg file (image files) can be mounted as a virtual dvd.
But I'm sure you are not interested in hair splitting.
Now that's out of the way, let's move on to the major one...
What you wrote in your initial post is about comparing I/O performance under Parallels of a VM based on Bootcamp versus a VM based on a Virtual Disk ("from within parallels"), and NOT comparing the performance running the Bootcamp partition natively (Booting the mac to Windows). Parallels can use a Bootcamp partition as the storage/disk of a VM.
And in that scenario you really don't need benchmarks to realise that if both OS X and the Windows VM, either in a Bootcamp partition or in a Virtual Disk/Drive, reside on the same physical disk they will be competing for I/O the moment they'll both want to read and write something from the disk at the same time, hence the slower performance.
Last edited: Oct 26, 2014