Using kvm debug linux kernel

I use kvm to debug the linux kernel, after set the breakpoint at start_kernel, the gdb can’t stop at start_kernel . I use the follow command:
sudo /usr/local/kvm/bin/qemu-system-x86_64 -drive file=Ubuntu10-10.img,cache=writeback,aio=native -kernel /home/src/linux-2.6.35/arch/x86_64/boot/bzImage -append root=/dev/sda1 -net nic,vlan=0,model=rtl8139,macaddr=52-54-00-12-34-01 -net tap,vlan=0,ifname=tap0,script=no -m 512 -s -S

gdb-7.2
gcc-4.5
kvm-0.13

I want to why ?