In some cases it's not possible to use a standard graphical terminal emulator to interact with a VM. In this case, "serial console" access is still available using two tools.
Usage: qvm-console-dispvm VMNAME
Launches a DispVM connected to the VM's console, using the qubes.ShowInTerminal RPC service. This provides a full-featured console.
At the time of writing this command contains a bug whose fix is waiting on release, therefore it may be necessary to use the following.
Usage: sudo xl console VMNAME
Uses Xenlight to directly access the VM console from dom0. For security reasons this console is deliberately limited in what it can display.
Line-by-line text will work fine, but if a Curses-style pseudo-graphical-interface comes up the output will be garbled and you will need a tool like asciinema to untangle it. You may need to substitute all dots followed by [, ], ( or ) to the ASCII ESC (0x1b) character, a method that could have false positives.