xen-pciback.hide is precisely removing the devices from the Dom0.

However, xen-cmdline --delete-dom0 will actually REMOVE the parameters you entered in the grub configuration file.

In other words, xen-cmdline is a helper tool to edit Grub configuration file for Xen and Linux arguments (on boot).

As it's written, xen-cmdline --set-dom0 will actually set parameters for the dom0, and xen-cmdline --delete-dom0 will remove them.