OpenVZ and Xen use different virtualization approaches. OpenVZ uses OS level virtualization, while Xen uses paravirtualization.  Those are quite significant differences between the two platforms.  OpenVZ fans feel it offers better resource usage, allowing users to burst RAM usage and use more than what is guaranteed and allocated to them.  While that is fine if your setup is RAM hungry and needing to hog the RAM on the server, but not so great for the other folks on the server.  This allows overselling the resources on the server, and for users to over use.  In a diverse user environment, this can mean performance issues when things get out of hand from running low on resources.

Xen offers good resource usage and superb isolation, better simulating a dedicated server environment.  Translation is your neighbor can't bring the whole server down.  This is one of the many reason we have opted for Xen on all of our Hybrid and virtual deployments.