What is the main different between the power transformer and voltage transformer?

Power transformer is used for transferring power from low voltage circuit to high voltage circuit and vice versa. They can be step up or step down transformer.

Voltage transformer are step down transformer with specific properties. It is used in metering and protection circuit for getting voltage proportionate to high voltage on its primary. As this transformer is used for metering and protection circuit, accuracy in transferring the high voltage to low voltage is of paramount importance.

as you saied ,power transformers have higher MVA than VT's or CVT's , connection of power transformer is different from VT ,there are three VT for one3ph ,each phases one VT , there are three or four output in VT's to connect to measuring center ,counters and protection relays . ratio of VT's are approximately1000/1 or100/1 for example in230kv the output of VT is100 volt .

There are several types of differences between the voltage and the power transformer.

One of the major difference between them is that the current transformer converts the high value of current into low value whereas the potential or voltage transformer converts the high value of voltages into low voltage.

In Each Electrical Transformer, We may Step Up or Step down the Voltage & vice versa the current. But no transformation of Power.

So there is no Transformer which may transform Power other then Voltage & Current Transformation.

But in practice, for operational & safety purcposes, High KVA rated transformers are also known as "Power Transformers"

In  the rating and cosequently the insulation level , power transformer ranges from low KVAs -to very high MVAs, and voltage transformer just few VAs as its loading is just measuring devices or protection relays.

Same concept but power transformer used for high power systems and voltage transformer used for stepping down voltages to low levels. It is just a matter of names

Voltage transformer is normally used for protection and monitoring purposes with fraction KVA ratings, where as power transformer is mainly used in transmission and distribution purposes, where the KVA rating will be large, normally in MVA.