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Engine Yard Cloud runs on AWS. AWS itself uses an in-house modified Xen.
Expanding on it a bit, Engine Yard pretty much provides a custom AWS EC2 AMI that will hook into their own control panel. It's Amazon that's actually hosting these images. Your payment to Engine Yard is a pass-through to Amazon with a bit taken off the top by Engine Yard.
If you bought a shared account from someone reselling Engine Yard, then it's still hosted at Amazon, but you have several more layers of indirect middle-man management.
Last edited by tchen; 12-10-2013 at 05:02 PM.
Reason: expanded info