The Xen 3.1 RPMs are pre-built RPM packages for a number of
different Linux distributions. For each distribution there
are three binary RPMs as well as a complete source RPM (SRPM). 

These RPMs can be used as input to the rpm tool to install Xen 
itself and all the necessary tools (xen-3.1.0*), as well as a 
Xen-enabled Kernel based on kernel.org 2.6.18 (kernel-xen-2.6.18-3.1.0*), 
e.g. 


# rpm -ivh xen-3.1.0-1.*.rpm
# rpm -ivh kernel-xen-2.6.18-3.1.0.*.rpm

Depending on your install, you may need to first install addtional
rpms (one common rpm not installed by default on a number of
distrbutions is bridge-utils for example). You may also need to 
remove conflicting rpms in some cases (e.g. some older distributions
may have installed rpms with a conflict against a kernel as recent
as 2.6.18). 

Note that when you install kernel-xen-2.6.18-3.1.0*, modules 
are installed and an initrd is built. However no new boot entry is
automatically added to grub.conf; instead you need to do this manually.
Typically this means adding an entry of the form 

title My Distro with Xen (2.6.18-xen_3.1.0)
        root (hd0,0)
        kernel /xen-3.1.0.gz
        module /vmlinuz-2.6.18-xen_3.1.0 ro root=/dev/<foo>
        module /initrd-2.6.18-xen_3.1.0.img

... obviously replacing <foo> with the name of your root device. 


If you plan to develop with Xen, download and install the 
xen-devel-3.1.0-1.* RPM to get the header files and
development libraries.


If you want the complete source for reference or to rebuild, 
download and install the SRPM (xen-3.1.0-1.src.rpm).