I'm guessing that all guests would be screwed, if the hypervisor is hacked. I found this article, but it seems to be more focused on privilege escalation of guests. I'm more interested to hear about other exploits on hypervisor managers. Many providers use VM control panels like SolusVM. Wouldn't it mean that a security vulnerability in the admin control panel would allow getting control of an array of machines?
Simple answer is yes. If an attacker controls the hypervisor they can potentially get access to everything within it, including the memory space of a VM (which could include keys)
Worst case is to assume all may be lost if the hypervisor is compromised, so in order to prevent this, a well structured, layered security architecture may make it incredibly difficult for an attacker to exploit the vulnerability.
One of the reasons we always encourage defence in depth!
All guest OSes can be affected.
This problem is very well explained in this presentation (from the 2008 RSA Conference).