Somebody has already installed at least part of XEN on the machine that already has Centos, and the installed version is NEWER than the one which comes along with Elastix 1.5. Either find a newer version of Elastix that will work with the xen-2.6.18-128.el5 or else you will have to downgrade the existing machine by removing XEN and reinstalling. Since this is part of the kernel it could be a painful process.
On the Elastix website, the latest version is 1.5.2. Is this what you are using?
I suspect that the installer asks you if XEN is already installed. If this is true, then you could try again and this time say "yes", however you will have two slightly different machines. An alternate route, and the one that I would use, is to manage XEN and the appliance completely separately. I would get both machines to have an up-to-date release of XEN, test that it is working, and then get the Elastix VMWare image and convert it to a XEN image.
qemu-img does a fine job at converting VMWare images.