According to European std. The domestic supply voltage is 220 V. When an equipment of high load current rating (Such as Kettle) is turned ON, the other elements such as 40W filament lamp is dim and also when checked any socket voltage with a DMM, it shows nearly 210 volts. How this happens..? In parallel domestic wiring, the voltages across any socket will be same(220). But how the heavy load affects the main supply of 220V..?
This indicates that you have a higher resistance in the service connection than there should be. I think you should get a service call from a competent electrician.
Probably your wires are thin and/or the line serving your appliance is long. Anyway the standard voltage is 230 +/-10% so anything between 207 and 253 is regular. Remember that voltage drop on a wire increase with the current so having a lower resistance (thicker) cable will reduce the voltage drop.
Usually for 16A circuits is used 2,5mm^2 (or 4mm^2) cable , for 10A circuit 1,5mm^2 (or 2,5mm^2). For house feeder (10kW) thickness should go from 10mm^2 (short run) to 25mm^2 (long run). Here is a calculator of voltage drop (should be <1,5%) for cables link If you don't understand Italian you can google-translate it. Between ( ) thicknesses if cable run is long