There are rare times when debugging problems in the Xen Project software may mean you will want to run QEMU under a debugger.
To run gdb on qemu-dm, replace the /usr/lib/xen/bin/qemu-dm by a script:
#!/bin/sh exec gdbserver 0.0.0.0:1234 /usr/lib/xen/bin/qemu-dm.bak $@
And run gdb. `target remote localhost 1234` to connect to gdbserver.
With recent versions of Xen Project software (4.1 and following), you can specify a different device model in the config file instead of replacing the default binary.