With certain guest operating systems, booting gives an immediate "Exception 13" or "Exception 12" message.

The problem is caused by the lack of real-mode virtualization on Intel processors. Real mode is the 16-bit instruction set used in the 8088 cpu, and is used for booting modern operating system, and by DOS. kvm uses a hack that allows most, but not all, of real mode instructions to be used. If your guest OS uses a real mode feature that kvm does not emulate, it will crash.

Many times it is possible to work around the issue by disabling the graphical boot found on many Linux distributions. For example, holding down the Shift key during guest boot up disables the graphical boot for some distros, tested with Ubuntu 7.10 desktop and KDE-Four-Live.i686-0.8.iso. It may be necessary to boot with the -no-kvm switch in order to get at the configuration files.

For Windows Vista: Make sure that the virtual machine has enough RAM (the command line switch is e.g. "-m 512" for 512 MB). The default setting (128 MB) causes Vista to reset with an "Exception 13" message.