Xen Performance/Overhead

I've tried asking on the xen-users mailing list, but haven't received much response. So, i'm asking here.

I'm running Xen 3.1 with CentOS 5 64bit on a Dell 2950 with 2 x 2.33Ghz Quad-Core CPUs. This should/is be a very powerful system. However, when running Xen the performance drop is huge. The strange thing is, on the mailing list others were reporting much lower levels of performance loss. (Just to be clear, I'm using the XenSource compiled kernel, etc.)

With Xen running, my UnixBench results aren't too bad.

Code:
INDEX VALUES
TEST BASELINE RESULT INDEX

Dhrystone 2 using register variables 376783.7 52116444.7 1383.2
Double-Precision Whetstone 83.1 2612.0 314.3
Execl Throughput 188.3 11429.1 607.0
File Copy 1024 bufsize 2000 maxblocks 2672.0 155443.0 581.7
File Copy 256 bufsize 500 maxblocks 1077.0 37493.0 348.1
File Read 4096 bufsize 8000 maxblocks 15382.0 1475439.0 959.2
Pipe-based Context Switching 15448.6 548465.7 355.0
Pipe Throughput 111814.6 3313637.0 296.4
Process Creation 569.3 34050.6 598.1
Shell Scripts (8 concurrent) 44.8 3566.8 796.2
System Call Overhead 114433.5 2756155.3 240.9
=========
FINAL SCORE 510.9
However, once I boot into Xen, the Dom0 performance drops a lot.

Code:
INDEX VALUES
TEST BASELINE RESULT INDEX

Dhrystone 2 using register variables 376783.7 50864253.7 1350.0
Double-Precision Whetstone 83.1 2617.9 315.0
Execl Throughput 188.3 2786.5 148.0
File Copy 1024 bufsize 2000 maxblocks 2672.0 159749.0 597.9
File Copy 256 bufsize 500 maxblocks 1077.0 44884.0 416.8
File Read 4096 bufsize 8000 maxblocks 15382.0 1191772.0 774.8
Pipe-based Context Switching 15448.6 306121.8 198.2
Pipe Throughput 111814.6 1417645.2 126.8
Process Creation 569.3 4699.2 82.5
Shell Scripts (8 concurrent) 44.8 781.6 174.5
System Call Overhead 114433.5 1021813.7 89.3
=========
FINAL SCORE 261.6
Now, here is where it gets weird. The only running DomU which is CentOS 5 PVed, gets a higher score than Dom0.

Code:
INDEX VALUES
TEST BASELINE RESULT INDEX

Dhrystone 2 using register variables 376783.7 38015133.3 1008.9
Double-Precision Whetstone 83.1 2023.4 243.5
Execl Throughput 188.3 3877.4 205.9
File Copy 1024 bufsize 2000 maxblocks 2672.0 270737.0 1013.2
File Copy 256 bufsize 500 maxblocks 1077.0 78470.0 728.6
File Read 4096 bufsize 8000 maxblocks 15382.0 1227115.0 797.8
Pipe Throughput 111814.6 1383157.5 123.7
Pipe-based Context Switching 15448.6 310378.3 200.9
Process Creation 569.3 7534.8 132.4
Shell Scripts (8 concurrent) 44.8 1179.6 263.3
System Call Overhead 114433.5 1056362.3 92.3
=========
FINAL SCORE 308.2

why the performance is so low? Perhaps any tips on boosting performance?