Ouch. Before reading this article I was seriously considering deploying a signing service as a HaLVM (Haskell) Xen PV unikernel running on EC2. The service would receive its private key after startup, such that the key never touches disk. Now I'm a lot less inclined to pretend that the Xen interface actually protects me...
Xen has had page-table and interrupt vector related security vulnerabilities. But I don't think EC2 would use non-ECC RAM, so I don't think it's vulnerable to this "rowhammer" technique. (I also don't think EC2 would do cross-VM page deduplication, another necessary condition.)
Afaik xen does not use memory deduplication. KVM aside, one should be worried about things running inside a linux host/vm, like containers. Maybe I am missing something