I run a Bootcamp partition with Windows 7 on it, and it can bitstream HD audio to my receiver. I have no experience in virtual machines for this.
From what I've seen, Windows 8 will not let you bitstream the HD formats either. And no, same goes for OS X.
You will also need to host your video files on external hd's, if you want to share the files between partitions.
When I read up on it, it was only possible to get Windows Bootcamp to stream the HD formats using Media Player Classic and _not_ XBMC, due to it needs to apply the codecs/plugins (forgot the names for these, sry - but they are mentioned in the install guides). XBMC will not bitstream HD audio from my Bootcamp partition, so I scrapped the idea of using it - dont know if this has changed.
The downsides for this are the weak look of standard Windows filesystem to manage your files, and the need to change frame rate on the receiver to 24fps when booting up in Windows 7 to watch movies, to avoid syncing and frame errors.