I need to produce external interrupt whenever there is a change in state of a pin. When the pin goes from high impedance state to ground or from ground to high impedance state, an interrupt must be generated. What is the possible way of doing it in AVR?
If you are talking about interrupting on the action of a normally open SPST switch, you can achieve something approximating this behavior by pulling the pin high through a resistor, and grounding through the switch. Then generate an interrupt on the change.
You should not interrupt on a floating pin. It's bad practice and you will get unpredictable behavior.