Usually Xen will try to sync the clocks of Dom0 and the DOMUs, however this might not work as expected. In order to make a DomU maintain the clock on its own, one has to set /proc/sys/xen/independent_wallclock to 1.
Upgrading Xen server from Ubuntu 7.10 to 8.04
Upgrading Xen DomU from Ubuntu 8.04 (hardy) to 10.04 (lucid)
Ubuntu 10.04 does not run with kernel 2.6.24, so install linux-server in Xen DomU and copy vmlinuz-* and initrd-* to Xen Dom0, so that the DomU can be booted with the new kernel.
Disk devices are now available as /dev/xvdaX instead of /dev/sdaX, so change root line in DomU configuration and entries in /etc/fstab of DomU.
In DomU configuration, change extra line from xencons=tty to console=hvc0. You also have to copy /etc/init/tty1.conf to /etc/init/hvc0.conf within the DomU and replace all occurrences of tty1 in the copied file with hvc0.
Using a special IP setup with Xen
See http://wiki.hetzner.de/index.php/9_IP_Adressen_mit_Ubuntu (German only)
Bad network perfomance in Xen DomU
If you are using a routed networking setup with Xen, you might encounter a bad network performance for the Xen DomU: The solution is to disable tx checksums for the virtual ethernet device: They are not needed as their are no physical errors on a virtual interface that only connects to another virtual interface in the Dom0. To disable tx checksums the following line can be added to /etc/network/interfaces
post-up ethtool -K eth0 tx off
This line has to be added within the definition of the corresponding interface and eth0 has to be replaced by the name of this interface.
Please note that this solution will not work with Ubuntu, if /usr is in the DomU is stored on its own partition: You will have to deactivate the ifup and ifdown hooks in /etc/udev/rules.d/85-ifupdown.rules because ethtool lies in /usr/sbin and /usr is not yet mounted when this hooks are executed. Deactivating this hooks probably won't hurt as the interfaces will be brought up by /etc/init.d/networking and you are unlikely to use a hotplugged interface within a Xen DomU. In newer Ubuntu releases (e.g. 10.04 - lucid lynx) this is not so easy, as the the network interfaces are managed by upstart. Therefore you have to copy ethtool to the root partion and use the corresponding path in the post-up script.