Boot without monitor

It's stupid but the latest Kubuntu versions won't allow you to boot into the console (without X Window), nor to boot without a monitor connected. If you disable or uninstall KDM it won't fully boot. Here is a workaround.

Create a file /etc/X11/xorg.conf with the following contents:

Section "Device"
Identifier "VNC Device"
Driver "vesa"
EndSection

Section "Screen"
Identifier "VNC Screen"
Device "VNC Device"
Monitor "VNC Monitor"
SubSection "Display"
Modes "1024x768"
EndSubSection
EndSection

Section "Monitor"
Identifier "VNC Monitor"
HorizSync 30-70
VertRefresh 50-75
EndSection

Open the file /etc/default/grub and edit this line:

GRUB_CMDLINE_LINUX="nomodeset"

Run the following command in order to take the above in effect.

update-grub

Now the machine will boot without a monitor.

If you wish to attach a monitor while the system is running, then do not do the Grub change above. In that case X will fail, but the system will boot and using Ctrl-Alt-F1 you can switch to a console.