As of now, there is no option of diesel engine in XL6. It is available in petrol variant only. The XL6 comes with a 1.5-litre, 4-cylinder petrol engine, perfectly paired with Suzuki's SHVS mild-hybrid system.
@HJ57 Earlier in the month of August 2019 there was some news about the MS XL6 being available in both petrol and diesel where the diesel motor would be borrowed from Ertiga. However, that did not happen. The car is only available with a petrol engine as of now. Maybe the option of a diesel engine will come later. The petrol engine is however BS-VI compliant.