Just a quick note for the search engines to find – peth0 can go missing from ifconfig if there is a GATEWAY= entry in ifcfg-eth0 (anaconda puts it here) and presumably -eth1, etc.. Put the default gateway in /etc/syconfig/network instead and use route-eth1 files instead to specify gateways.
Reboot for xend to do its setup correctly (please commment if there’s a way to do this without reboot that works…) and instead of ‘no peth0’ you’ll find one now exists. Also, your xenbr0 will be set up properly.