I have OVM 3.2.1, i instaled two VMs Oracle Linux 6.4 one VM with HVM Domain type and the second PVM Domain Type. And for both of them, i can edit VM configuration and add new virtual disk when the VM is running (verify with ls /dev/xvd*), And Oracle Linux 6.4 use by default the LVM so you can increase the size of the disk also when the system is running.
I hope this can help you
If you change the size of the vdisk in OVMM, you guest won't notice that until the device get's unplugged and plugged in again. So, if you want to resize a vdisk, that is in use by your guest, you will likely have to restart it, should it contain the running kernel.
However, as stated above, if you're using LVM, you can plug in a new vdisk and use that to resize the lv(s) on your guest.