The following is from the OL 6.5 release notes:
Hardware virtualized guest with kernel-uek-2.6.32-100.35.1.el5uek and higher
Unbreakable Enterprise Kernel version kernel-uek-2.6.32-100.35.1 adds support for paravirtualized drivers in a harware virtualized guest on Oracle VM. Starting this kernel version hardware virtualized guests which are not configured to use paravirtualized drivers will need an additional kernel boot parameter "xen_emul_unplug=never" in /etc/grub.conf:
kernel /vmlinuz-2.6.32-100.35.1.el5uek ro root=/dev/VolGroup00/LogVol00 xen_emul_unplug=never
With kernel-uek-2.6.32-100.35.1 and higher, the default is to present only paravirtualized drivers when running in a hardware virtualized guest on Oracle VM. Adding this kernel boot parameter makes the kernel also present the emulated drivers as it did before (e.g. the 8139cp network driver may no longer load if this parameter is not set).
Older version of Unbreakable Enterprise Kernel do not have support for hardware virtualized guest that uses paravirtualized drivers (PVHVM).