Is there any visible difference between thyristor and transistor?

A diode is a solid state passive device that will only allow current to flow in one and only one direction (Anode->Cathode). A Thyristor behaves exactly like a Diode except that in order for current to flow it must "see" current at its gate, if it does not see current (ie logic 0 or ground) then current is not allowed to flow from its anode to cathode.

Note: The Thyristor gate requires little current to trigger its gate, meaning a small amount of current can be used to switch on/off a load require much more current.

Another Note: Since the Thyristor has a gate which controls its output it is not a passive device like the diode