you need to specify a logfile:
something like
-l /var/log/tvheadend.log
Then you can read the log file in real time like so:
tail -F /var/log/tvheadend.log
The log settings are specified from the GUI, you need to be in Advanced or Expert level though:
Configuration -> General -> Base -> User Interface Level
set to "Advanced" or "Expert"
Configuration -> Debugging -> Configuration