For example, you want to use a tape device in a domU. Therefore you need a PCI adapter, which is not used in the dom0. Then pass this device (entirely) to the domU.
First find out the numbers.
Second, use pciback.hide in grub.conf (on you dom0) to hide your SCSI card.
title Debian dom0 (2.6.18xen) root (hd0,0) kernel /xen.gz-2.6.18-8.1.4.el5 module /vmlinuz-2.6.18-8.1.4.el5xen ro pciback.hide=(02:0b.0) module /initrd-2.6.18-8.1.4.el5xen.img
And then allow access to the PCI card in your domU configuration file:
kernel = "domU-current" memory = 256 name = "backup01" vif = ['ip=192.168.1.2] pci = [ '0000:02:0b.0' ] dhcp = "dhcp" disk = ['phy:/dev/vg/disk01,sda1,w', 'phy:/dev/vg/disk01.swap,sda2,w'] root = "/dev/sda1 ro"