If you want to follow this howto to the letters, get Xen 3.0.2 even though this howto said Xen 3.0.1.

 I have tried both versions. 3.0.1 not only has slightly different version of Linux kernel, but it didn't work at all no matter how I tweaked it accordingly (to this howto and to the Ubuntu 100 hacks book). For one example, in the /lib/modules directory, it contains both a Xen0 and XenU directory. In later versions, this has simplified to just a Xen directory.