Red Hat Enterprise Linux 3 is uses processor architecture specific kernel RPMs and because of this the para-virtualized drivers may fail to load if the para-virtualized driver RPM does not match the installed kernel architecture.

When the para-virtualized driver modules are inserted, a long list of unresolved modules will be displayed. A shortened excerpt of the error can be seen below.

insmod xen-platform-pci.o 
                    Warning: kernel-module version mismatch
                    xen-platform-pci.o was compiled for kernel version 2.4.21-52.EL
                    while this kernel is version 2.4.21-50.EL
                    xen-platform-pci.o: unresolved symbol __ioremap_R9eac042a
                    xen-platform-pci.o: unresolved symbol flush_signals_R50973be2
                    xen-platform-pci.o: unresolved symbol pci_read_config_byte_R0e425a9e
                    xen-platform-pci.o: unresolved symbol __get_free_pages_R9016dd82
                    [...]

The solution is to use the correct RPM package for your hardware architecture for the para-virtualized drivers.