difference between Photodiode , Varacter diode , Tunnel diode

A photodiode is similar to battery that produces current in the forward directions when light shines on the photodiode.

A varacter diose is similar to capacitor/condenser whose capacitance changes with reverse voltage across it. A varacter diode is used to adjust a tuned circuit resonant frequency by changing the DC voltage across diode. 

A tunnel diode is similar to an ordinary diode where except that it has a -Ve resistance over part  of the VI forward characteristics.

These are the differences between three of them/ each has its unique applications and can be used within one circuit board or within multiple.