Try VirtualBox from Sun/Oracle (just for converting the image). It can convert many image-types. Read the fine manual about how to do so using the cli. You can use the RH-rpm-version of VirtualBox for CentOS.
You should be able to import the ovf file directly into Xen. With XenCenter, there is an option in the menus for to choose an ovf when you import. You might also be able to use the Xen Converter from Citrix to do it, if XenCenter isn't an option for you.