The easy way is to add tvheadend too your /etc/rc.local. The line would look something like: tvheaded -f -u (user to run it as) -g (group).
The other solution would be to create a script like this one in /etc/init.d:
### BEGIN INIT INFO
# Provides:          tvheadend
# Required-Start:    $remote_fs $syslog userhdhomerun
# Required-Stop:     $remote_fs $syslog
# Default-Start:     2 3 4 5
# Default-Stop:      0 1 6
# Short-Description: Start tvheadend daemon at boot time
# Description:       Enable service provided by tvheadend daemon.
### END INIT INFO
TVHNAME="tvheadend"
TVHBIN="/usr/bin/tvheadend"
TVHUSER="hts"
TVHGROUP="hts"
case "$1" in
  start)
    echo "Starting tvheadend"
    sleep 5
    start-stop-daemon --start --user ${TVHUSER} --exec ${TVHBIN} -- \
                -u ${TVHUSER} -g ${TVHGROUP} -f -C
  ;;
  stop)
    echo "Stopping tvheadend"
    start-stop-daemon --stop --quiet --name ${TVHNAME} --signal 2
  ;;
  restart)
    echo "Restarting tvheadend"
    start-stop-daemon --stop --quiet --name ${TVHNAME} --signal 2
    start-stop-daemon --start --user ${TVHUSER} --exec ${TVHBIN} -- \
                -u ${TVHUSER} -g ${TVHGROUP} -f -C
  ;;
  *)
    echo "Usage: tvheadend {start|stop|restart}"
    exit 1
esac
exit 0
Give it the right permissions with chmod 755 and then with the command "insserv nameofyourscript" make it run everytime you boot your server.