Nm stands for Newton Metre.
Kg-m stands for Kilogram Metre.
Nm multiplied by a factor of 0.102 gives you the Kg-m figure. Both are measures of torque.
HP = (Torque x RPM)/5252.
Diesel engines are usually turbocharged while their petrol siblings are naturally aspirated and they also run way higher compression ratios than petrols. Hence the higher torque on diesels.
This is the simplest possible explanation.