Você pode usar o Xen e o Remus. O Remus mantém uma cópia ao vivo da sua VM que assume o controle se a cópia original morrer em você.
De link :
Remus provides transparent, comprehensive high availability to ordinary virtual machines running on the Xen virtual machine monitor. It does this by maintaining a completely up-to-date copy of a running VM on a backup server, which automatically activates if the primary server fails. Key features:
- The backup VM is an exact copy of the primary VM. When failure happens, it continues running on the backup host as if failure had never occurred.
- The backup is completely up-to-date. Even active TCP sessions are maintained without interruption.
- Protection is transparent. Existing guests can be protected without modifying them in any way.
É um recurso recente no Xen, portanto, pode exigir alguns ajustes, mas é grátis!