• create a 'iso bootable image without legacy floppy emulation' on rom-o-matic for networkchip rtl8139
• some other network-chips supported by the xen-qemu-code also work for pxe, generate for different networkchip && specify model in the xen-vm-configfile
• the xen-vm-configfile i use:

import os, re
arch = os.uname()[4]
if re.search('64', arch):
    arch_libdir = 'lib64'
else:
    arch_libdir = 'lib'
kernel = "/usr/lib/xen/boot/hvmloader"
builder='hvm'
memory = 512
name = "HVMpxestart"
vcpus=1
acpi=1
apic=1
vif = [ 'type=ioemu,bridge=xenbr0,mac=aa:00:00:00:00:27',
        'type=ioemu,bridge=xenbr0,mac=aa:00:00:00:00:28',
        'type=ioemu,bridge=xenbr0,mac=aa:00:00:00:00:29' ]
disk = [        'file:/mnt/lv_public/xen_testimg,hda,w', 'file:/etc/xen/eb/eb-5.4.3-rtl8139.iso,hdc:cdrom,r' ]
device_model = '/usr/' + arch_libdir + '/xen/bin/qemu-dm'
boot="dc"
sdl=1
vnc=0
stdvga=0
soundhw='none'
usb=0
keymap='en'

• works for qemu 0.9, maybe xen can do pxe directly once this code gets merged: "qemu -hda debian_mini -net nic,vlan=0,model=pcnet -net tap,vlan=0,ifname=tap0 -boot n" (works for all three kinds of qemu network-cards, just having to restart the dhcpd after creation of the tap-interface)
• other idea for xen, not investigated: boot usual kernel from disc, and kexec (with kboot) into new one from network via pxe