Sensitivity: The sensitivity of a voltmeter is just its capability to detect voltage, usually meaning smaller and smaller voltages. When selecting a meter for a certain voltage measurement, it is important to consider the sensitivity of a dc voltmeter, A low sensitivity meter may give a correct reading when measuring voltage in a low resistance circuit. but it is certain to produce unreliable readings in a high resistance circuit.
Loading Effect: The voltmeter is always connected across the two points between which the potential difference is to be measured. If it is connected across a low resistance then as voltmeter resistance is high, most of the current will pass through a low resistance and will produce the voltage drop which will be nothing but the true reading. But if the voltmeter is connected across the high resistance then due to two high resistances in parallel, the current will divide almost equally through the two paths. Thus the meter will record the voltage drop across the high resistance which will be much lower then the true reading. Thus the low sensitivity instrument when used in high resistance circuit gives a lower reading then the true reading. This is called loading effect of the voltmeters. It is mainly caused due to low sensitivity instruments.