Look, I don't understand entirely why they're using VMs and what all the specifics are.
Besides the resource sharing and security benefits (which are the reasons i've been given) it could also facilitate easy BC for the next Xbox (if there is one).
Or allow you to play Durango games on your PC or Surface etc
Or allow a transition to a cheaper to manufacture hardware design later on, as liolio was speculating.
The VMs can talk to each other through the hypervisor (Host OS) which handles inter-OS communication and all hardware access, and hosts the other two OS instances (Title and System) in VMs.
The Host and Title OS (which the game runs in) are both stripped down (similar to the 360 kernel) and as mentioned before, they have Dave Cutler working on low overhead virtual drivers and stuff to optimise VM performance for games.
The other System OS runs the Win8 kernel modified in a similar way to Windows Phone 8, allowing only RT stuff, with no desktop capabilities.
So i'm guessing parties, voice chat etc reside in the System OS with data passed to/from the title OS as required.
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