Difference CentOS Xen and Citrix XenServer
 

What is the difference between the xen-sources in the repo of CentOS and installing Xen that way, and installing Citrix Free XenServer 5.5.0 ??

Citrix Free XenServer 5.5.0 is a bare metal hypervisor. Guest OSs will perform better in citrix. It has a nice management interface and if you have a windows box you can use Xen Center to remotely manage the Xen Server and the VMs. Xen Center has a lot of powerful features and makes managing vms easy. You need a 64bit PC to be able to run Citrix Xen Server.
The other one on CentOS just sits on top of your operating system and provides virtualization using qemu. Mostly you would use the command line tools like virt-install and xm to manage the vms. You can install it on 32bit/64bit both architectures.

You will need a seperate 64bit computer to run the Citrix Xen Server because you cannot run it on top of your existing OS. For the xen, you can install and use it as an application in your existing centos installation.

Thanks you for your reply.

Have you experience with virt-manager ? Would this interface bring me close to the Citrix management module ?

Are you telling me that I can not remotely manage the Citrix Xen Server when I don't use a Windows desktop ?

Yeah, you can use virt-manager but its nothing close to Citrix Xen Center. But it'll let you do the basic stuffs.
Yeah you cannot remotely manage citrix xen server using xen center without a windows box. While there are other ways to remotely manage the xen server but its not as efficient as using a xen center. You can ssh into the xen server box and manage it using the xe command or using the xconsole interface.
If you are just learning and testing stuffs, you don't necessarily need the citrix xen server.

Well, it is indeed a setup to learn Xen. But I like to begin with something good right away.

Always used VirtualBox before...

Yeah learning is fun and its always a good idea to begin with the best.
So have a separate 64bit box for xen server with at least 2 gigs of RAM. and then use your laptop or another windows computer to connect to it using xen center. Please note that the xen server will wipe the entire hard drive on the system. Also you need to setup a network between the xen server and the windows computer used for xen center.