sysrq

Send Magic SysRq to a KVM guest using virsh

virsh send-key guest1 KEY_LEFTALT KEY_SYSRQ KEY_H

Kernel Stack Traceback

If you have hung processes, capture a stack traceback to show where the processes are waiting in the kernel. You will need to build your kernel with the CONFIG_MAGIC_SYSRQ option (under Kernel Hacking) to enable stack traceback.
First, look in /etc/sysctl.conf to see if kernel.sysrq is set to 1. If not, then run this command:

echo 1 > /proc/sys/kernel/sysrq

Next, trigger a stack traceback via this command:
echo t > /proc/sysrq-trigger

Look on your console or in /var/log/messages for the output.