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Diesels will generally give better fuel economy than petrol (gasoline) units.

Fuel consumption varies PRIMARILY by kW - real power.

Another big issue is percentage of unit rating. Typical published fuel consumption is at full or near full loading, Very lightly loaded engines will have much poorer fuel consumption per kWHr as mechanical losses remain about the same, since RPM doesn't change with loading %. So fuel consumption isn't fully proportional to loading.

Generators/Alternator ratings of kVA is based on amp current limits in windings (keeping temperature within limits of insulation). But kW rating is dependent on available horsepower of prime mover. Output frequency is dependent on RPM of prime mover and number of poles in the alternator design.

Generally generators are rated at 0.8 power factor, but within their kW limits can safely power even at unity power factor, as winding current is lower at unity power factor. So overloading on kVA will show as alternator section too hot and shorten winding life. Overloading on kW will tend to drag down engine RPM, this can happen due to high altitude or high ambient temperature giving a reduction in available horsepower (clogged air filters also!).

I've got a document of items to consider when using generator as a prime power source.