not with XEN
XEN can not run XEN, meaning that you cannot run a VM host inside another VM.
KVM CAN do this. you can run 5 servers inside KVM on one box, then inside those 5 VMS you can run 5 more servers under KVM.
25 servers on one box might seem excessive, but that and more is possible.
Also, KVM in RH5.4 or Centos5.4 is a bit faster overall than Xen, though Xen gives faster access to devices so network and I/O will be a bit faster in Xen. KVM can pass PCI devices to virtual machines, like network cards or storage devices like SATA controllers which would make the VM have NATIVE speeds off those devices.