Please take a look at the section Operation System booting.
The most common option is the BIOS bootloader where it allows you specifying boot order via changing domain XML. try command virsh edit <your-domain-name>
then adjust the boot order with this example (copied from libvirt.org):
<os>
<type>hvm</type>
<loader>/usr/lib/xen/boot/hvmloader</loader>
<boot dev='network'/>
<boot dev='cdrom'/>
<boot dev='hd'/>
<bootmenu enable='yes'/>
</os>
Which means the boot order is:
- boot from network like PXE boot
- boot from CD-ROM if no PXE boot
- boot from local hard disk if no PXE boot nor bootable CD