linux - Ubuntu 10.04 - high load , low cpu, IO, and no swapping

Ubuntu 10.04 64bit - Kernel 2.6.36
Running as a Xen 3.4 guest.

8GB memory , no swap being used, no IO wait time, and 0% cpu.
Yet high load. Why is this:

15:11:24 up 5 days,  3:22,  2 users,  load average: 21.23, 20.06, 19.51
Tasks: 113 total,   1 running, 112 sleeping,   0 stopped,   0 zombie
Cpu(s):  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
Mem:   7416372k total,  5250472k used,  2165900k free,   153260k buffers
Swap:  1959924k total,    26288k used,  1933636k free,   246932k cached

IO top shows nothing.

free -mem

        total       used       free     shared    buffers     cached
Mem:          7242       5127       2115          0        149        241
-/+ buffers/cache:       4736       2506
Swap:         1913         25       1888