To run Ubuntu 10.04 32-bit and 64-bit servers under Citrix XenServer 5.6, which kernel should I use for the best performance? For new installations, can I specify that kernel in the boot option for the installer?
The "virtual" kernel leaves out drivers that aren't useful in a VM - i don't know that it increases performance as such, but it'll be a bit smaller. To choose it, make sure you're booting the "Server" CD / ISO and at boot-time (ie. before you hit return to "Install Ubuntu Server), hit F4 ("modes") and choose "Install a minimal virtual machine".
If you set up PXE boot and a preseed file, you can tell it to install the "virtual" kernel by adding the line
Alternatively, you could use the "netboot" mini iso (about 13MB) which provides enough of an installer to fetch the rest of the system from the internet - with this and the PXE option, if you have some kind of proxy like