Stability of Xen ?

I just looked at the reboot posts for the past three months:

      atlanta35: Xen bug
      atlanta46: unresponsive
      dallas44: ThePlanet powercycled!
      dallas47: Older Xen bug; Xen updated
      dallas73: ThePlanet powercycled!
      dallas74: unresponsive
      dallas77: frozen
      dallas78: unresponsive twice; migrated off hardware ?
      dallas83: Xen bug
      host56: Older Xen bug; Xen updated
      newark6: unresponsive; hardware failures?
      newark12: Xen bug
      newark22: Xen bug
      newark37: unresponsive
      newark41: unresponsive; Xen stack updated
      newark45: unresponsive

Are all of those machines Xen based? Or are any UML based? Has any cause been determined for the "unresponsive" failures?