When I run xm list, it shows some thing like this:
Name ID Mem(MiB) VCPUs State Time(s)
Domain-0 0 4551 2 r----- 1835.7
windows2003server 1 1031 2 -b---- 4056.9
Why the domU's running time can be bigger than Dom0's?
because the time field in the xm(1) command gives you the CPU time used by a given domain as accounted by Xen, not the elapsed time since the domain was started.