You can use xm to migrate a domain:

 
xm migrate [domain-id] [host] [options]

Where [domain-id] is the domain you want to migrate, and [host] is the target. The [options] include ——live (or -l) for a live migration, or ——resource (or -r) to specify maximum speed of the migration (in Mbs).

To ensure a successful migration, you must ensure that the xend daemon is running on all hosts domains. All hosts must also be running Red Hat RHEL 5.0+ and have migration TCP ports open to accept connections from the source hosts.