How to increase CPU core speed of a guest to maximum of Citrix Xenserver Host

There is a Citrix XenServer 5.5 (HP ML370 G5, Xeon 2GHz) on this Server there is a Windows Guest (Windows 2008 R2 x64) under Properties I set 4 CPU Cores with max prio. But if I check with CPU-Z the performance of the CPU from Host there is set the right CPU (XEON 2GHz) but with only 600MHz what is wrong with this Guest.

The /proc/cpuinfo from Citrix XenServer:

processor : 0
vendor_id : GenuineIntel
cpu family    : 6
model     : 15
model name    : Intel(R) Xeon(R) CPU           E5335  @ 2.00GHz
stepping  : 7
cpu MHz       : 2000.016
cache size    : 4096 KB
fdiv_bug  : no
hlt_bug       : no
f00f_bug  : no
coma_bug  : no
fpu       : yes
fpu_exception : yes
cpuid level   : 10
wp        : yes
flags     : fpu de tsc msr pae cx8 apic mtrr cmov pat clflush acpi mmx fxsr sse sse2 ss ht constant_tsc up pni vmx
bogomips  : 4002.63

Here is the Outout of CPU-Z from Windows Guest:

On another Citrix XenServer under Citrix XenServer 6.1 with the same machine it works fine. Is this a failure from Citrix Xenserver or of HP Hardware?

Update The Core speed don't increase if the CPU has load:

Update 2 This is the Output of xenpm

# xenpm get-cpufreq-states
Xen cpuidle is not enabled!
Xen cpufreq is not enabled!