OpenVZ has one strong limit compare to XEN, it is not a full visualization and therefore you're limited to Linux only containers. People working with Sun will recognize Solaris zones concept, that was introduced few years ago. Like for Solaris every OpenVZ zones shared the same kernel, which at OVH translate in a Linux-188.8.131.52 kernel. This being said, it is important to understand that Linux distributions are independent of kernel, you can therefore run any Linux distributions you want under a unique kernel. While OVH ships Debian Etch with OpenVZ hyperviseur, you can chose any other distribution for your zones, new version of Fridu mostly operated with Ubuntu, but nothing prevents you from running multiple distributions. OVH ships template for Debian, CentOS, Gentoo and Ubuntu, but if this is not enough you can either create your own template or download one from Internet (OpenVz-WIKI)
OpenVZ includes a set of scripts to create/manage virtual machines, unlike Xen that is shipped naked and where I had to write more or less equivalent scripts by myself (cf: Fridu Xen Quick Start). Furthermore OVH ships OpenVZ with a web console from Proxmox, not that I'm a big fan of having a GUI, but as you can see on the video, it is great to make sexy demos.This console allows you to create a new virtual instances literally in a mater of seconds :) It allows you to start/stop change ram size, IP adresses, etc. on any instances without forcing you to remember any special commands. While Proxmox console misses few features like an SSH applet, a firewall config, or a java VPN. I must say that I get used to it and create every virtual machine through the web GUI.
OpenVZ is very light weight, not only it shares the same kernel, but also the same filesystem and networking stack. Direct result is that, on a given server you can run more OpenVZ zones than you could run XEN virtual-machines. From a user point of view when a zone is up, wether you run OpenVZ or XEN is fairly transparent, this being said they are nevertheless some fundamental differences:
Tuesday, December 02, 2008
A Nice Xen vs. OpenVZ Comparison
Why OpenVZ and not XEN has a nice summary of some of the differences that are relevant to some of the comments made in response to OpenVZ fever