The open source version Xen may only run 32bit DSS in your VM, so try that as well.
Also, typical opensource VM's can, on occasion, emulate hardware improperly, and instead load generic drivers for things like NICs. Make sure you have the support you need for this, in CentOS.
And as To-M said, how much RAM ? .... I would go for at least 2GB as well, if not 4GB.