[CentOS] centos5/xen does not see all memory

I have a server with 6 gigs of memory. CentOS5/Xen sees only half of it. Could someone advice me how to get it recognize all my memory?

I have installed 32bit CentOS5 and I'm running kernel-xen-2.6.18-8.1.8.el5. This kernel should have PAE support:

$ grep PAE /boot/config-2.6.18-8.1.8.el5xen CONFIG_X86_PAE=y

But xentop reports only 3 gigs of memory:

Mem: 3145148k total, 2400512k used, 744636k free CPUs: 2 @ 2612MHz

If I boot with kernel-PAE-2.6.18-8.1.8.el5 the system recognizes all memory but I cannot use Xen.

My motherboard is Supermicro H8SMA-2 A64/Opteron with MD Athlon 64 X2 5200+ processor and 2 x 1GB ECC DDR-II SDRAM and 2x 2GB ECC DDR-II SDRAM.

Regards, Peter