dom0:

thulium:~#  time (dd if=/dev/zero of=/tmp/testfile bs=1024000 count=500;sync;sync)
500+0 records in
500+0 records out
512000000 bytes (512 MB) copied, 11.1825 seconds, 45.8 MB/s

real    0m19.136s
user    0m0.000s
sys     0m1.180s

domU:

ytterbium:/home/otto# time (dd if=/dev/zero of=/tmp/testfile bs=1024000 count=500;sync;sync)

500+0 records in
500+0 records out
512000000 bytes (512 MB) copied, 85.5189 seconds, 6.0 MB/s

lutetium:/home/otto# (dd if=/dev/zero of=/tmp/testfile bs=1024000 count=500;sync;sync)
500+0 records in
500+0 records out
512000000 bytes (512 MB) copied, 92.3894 seconds, 5.5 MB/s

another domU in same dom0: