Here is more infos !
With a -current source tree, recompiled today (1st August), I have a 
more complete output and can access to the debugger :
[root@xenroot ~]# xm create -c webserver
Using config file "/etc/xen/webserver".
Started domain bsdweb
Loaded initial symtab at 0xc0748fbc, strtab at 0xc0765110, # entries 7120
Copyright (c) 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 
2005, 2006
    The NetBSD Foundation, Inc.  All rights reserved.
Copyright (c) 1982, 1986, 1989, 1991, 1993
    The Regents of the University of California.  All rights reserved.

NetBSD 3.99.24 (INSTALL_XEN3_DOMU) #0: Wed Aug  2 00:03:58 CEST 2006
        root@gedeon:/export/src/sys/arch/i386/compile/obj/INSTALL_XEN3_DOMU
total memory = 128 MB
avail memory = 118 MB
mainbus0 (root)
cpu0 at mainbus0: (uniprocessor)
cpu0: AMD Duron (686-class), 1294.49 MHz, id 0x671
cpu0: features c3c3fbff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR>
cpu0: features c3c3fbff<PGE,MCA,CMOV,PAT,PSE36,MMXX,MMX>
cpu0: features c3c3fbff<FXSR,SSE,3DNOW2,3DNOW>
cpu0: I-cache 64 KB 64B/line 2-way, D-cache 64 KB 64B/line 2-way
cpu0: L2 cache 64 KB 64B/line 16-way
cpu0: ITLB 16 4 KB entries fully associative, 8 4 MB entries fully 
associative
cpu0: DTLB 32 4 KB entries fully associative, 8 4 MB entries 4-way
cpu0: 8 page colors
hypervisor0 at mainbus0
debug virtual interrupt using event channel 3
xenbus0 at hypervisor0: Xen Virtual Bus Interface
xencons0 at hypervisor0: Xen Virtual Console Driver
xencons0: console major 143, unit 0
xencons0: using event channel 2
npx0 at hypervisor0: using exception 16
Xen clock: using event channel 4
raidattach: Asked for 8 units
Kernelized RAIDframe activated
md0: internal 5000 KB image area
panic: HYPERVISOR_mmu_update failed

Stopped in pid 0.1 (swapper) at netbsd:cpu_Debugger+0x4:        leave
db>

So I can type some command to help tracking the bug (but I'm a noob 
about this).