Understanding Xen

Alright because of my little issue with virtualbox on my spare machine where I use a tripleboot of opensuse, ubuntu and XP I have installed Xen to see what it is like.
But I cant get my head around it, firstly it asks me if I have an OS preinstalled (obviously I have one) or do I need to install one.
I usually use a VM to test out new distros and such but the way Xen makes it look that if say I run Mandriva in Xen it will actually install mandriva for real.
I am quite unsure of the info xen gives me, I looked around a bit for a guide but nothing really helpful.

err help?
It will be appreciated

Well,with my little knowledge,Xen is the not the most easiest Virtualization technology.But still,u can run a New distro(like Ubuntu (9.04) as Vm without installing it,if u have Cd/DVD which has the OS already installed it.Having said that,Even i`m struggling to build XEn manually in my computer(open suse 11.1)