Why kelvin scale starts from 273?

Ah, this is a little confusion, I think. The Kelvin scale does not start from 273. It starts at 0 K. This is ‘absolute zero’. It is the most cold possible. At this temperature, there is no more movement of atoms.

The Celsius scale is different. It is made for water. So, 0 degrees Celsius is when water becomes ice.

This same point, the freezing of water, is 273.15 on the Kelvin scale. So the scales just have a different start point. One degree of Celsius is the same size as one Kelvin.