I have installed NetBSD/i386 5.0.1 and sysutils/xenkernel33. Then I've got following:

(XEN)  -> Using new ACK method
(XEN) checking TSC synchronization across 2 CPUs: 
(XEN) CPU#0 had 0 usecs TSC skew, fixed it up.
(XEN) CPU#1 had -1867377 usecs TSC skew, fixed it up.
(XEN) Platform timer is 25.000MHz HPET
(XEN) Brought up 2 CPUs
(XEN) I/O virtualisation disabled
(XEN) *** LOADING DOMAIN 0 ***
(XEN)  Xen  kernel: 32-bit, PAE, lsb
(XEN)  Dom0 kernel: 32-bit, lsb, paddr 0xc0100000 -> 0xc0a50c54
(XEN)  Dom0 symbol map 0xc0a50c54 -> 0xc0b1d488
(XEN) Mismatch between Xen and DOM0 kernel
(XEN) 
(XEN) ****************************************
(XEN) Panic on CPU 0:
(XEN) Could not set up DOM0 guest OS
(XEN) ****************************************
(XEN) 
(XEN) Reboot in five seconds...
(XEN) Resetting with ACPI MEMORY or I/O RESET_REG.

This means 5.0.1 has NO PAE enabled kernel. xenkernel 3.3 has PAE enabled.
You may use 5.1 + xenkernel 3.3 or 5.0 + xenkernel 3.1.x
Thanks Fukaumi San.
Above console was captured with this boot.cfg. Here com1 is not typo, com0 and com1 combination might be important.