Put really simply a zener barrier (generally) is a passive device that consists of a resistive element with a zener diode to limit the output energy, where as a galvanic isolator is an electronic device that uses galvanic isolation as the protection.

There are a number of good documents on the internet covering the basics of barriers, isolators and intrinsic safety.

http://www.slideshare.net/rlaverne/lunch-and-learn-presentation#

http://en.wikipedia.org/wiki/Electrical_equipment_in_hazardous_areas

http://en.wikipedia.org/wiki/Intrinsic_safety

http://www.omega.com/techref/intrinsic.html

http://www.mtl-inst.com/support/faq_intrinsic_safety

Also try an internet search of the following search strings "zener barrier", "galvanic isolator", "intrinsic safety" and "basics" or "comparison". It is a very large field and so there is plenty of information available.

Also what are the voltages available at their terminals at different conditions for e.g. for DI barrier what would be the voltage at energized and de energized condition, similarly for DO, AI and AO. We are using both MTL and P&F barriers and isolators

It depends on the application and the intrinsic safety standards. Have a look at the MTL and P&F datasheets and website along with the datasheets and websites of the equipment you are connecting up to the barriers or isolators for more information.

purpose of the barriers and Isolator are same to keep safe energy level in hazardous zone ,apart from this similarity one can say that barriers are for intrinsic safety wiring purposes.and Isolators see many uses including splitting signals between a field device and DCS +PLC AND connecting an output from one equipment to another.

DI/DO depends on what type of device it is connected with the barriers,also if the device loop have a line control where we get open,normal and shortckt indications on barriers .same is with AI/AO . even though the supplied voltage is 24vdc when check on enerzised and de engerzised condition voltage level change between 4 to 8 vdc for DI/O in case of P&F proxy switch barriers . check with the vendor to get the exact specification