Just because I always forget and the parameters are somewhat non-trivial, here’s how you start an xterm inside X:
startx $(which xterm) -- :0 vt01
Of course, the display name
:0 is usually used by default, so you may need to use another one (
:1 etc.) if you get an error like “Server is already active for display 0“. You can also select a different virtual terminal.
Note that the absolute path to xterm (or whatever application you’d like to start inside X) is essential.