The VM Server log files are stored in the /var/log/xen directory on each VM Server:
domain-builder-ng.log Logs the creation of domains (virtual machines).
xend.log Logs all of the Oracle VM Server deamon actions. This log is very detailed and contains a lot of information. This same information can be retrieved via the xm log command.
xend-debug.log Logs some limited debug and trace information related to the Xen server daemon. ...