In a terminal:
sudo update-alternatives --config x-terminal-emulator
Select konsole from the list of alternatives (should be number 1 – assuming you like konsole). This makes konsole your preferred x-terminal-emulator which will be used when you press ctrl-alt-t to open a terminal and for any other launcher or menu entry that is configured to open a “terminal” without specifying which one.
There are also other terminal apps besides xterm and konsole; for example, mrxvt (available via Debian package installation system) will allow you to have multiple tabs, and is generally 'lighter weight' (less RAM when run) than both konsole and the default terminal emulator, xterm.