Thanks pixellany and rayfordj..
rayfordj, my doubt is, even if I skip the installation key during installation, I can install those packages later from the DVD via yum or rpm utilities, is it not?
so, is it something like this:
If you enter the installation key during installation, you have the option to install xen and packages related to clustering during installation itself; if you skip it, you only need to install them later from the DVD. Is this correct?