does xen improve security?

Hi all, imagine a ftpserver and a webserver running in two xen virtual machines on one box; now say the ftpserver is hacked, the attacker gains root privileges; How much does xen protect the other vm (webserver) against the attacker? Is it as if the webserver was running on a different physical box? In /usr/share/doc/packages/xen the main focus seems to be flexibility rather than security. Anyone with experience? Thanks, Joe