There are a significant number of ways in which a hybrid can achieve a big reduction in fuel consumption and perhaps more importantly CO2 Emissions. I can talk about the system that Ford and Toyota use (they use a common technology) I had both a Ford Kuga PHEV and a Kuga diesel. The PHEV was great, significantly better than the diesel even if I didn't charge it. Charging gave me a 30 mile electric range with a smallish battery. Not all hybrids run this way and so it does vary from manufacturer to manufacturer, although there are common themes. Biggest points are

1:  the Ford and Toyota Engine is modified to have a very long expansion stroke (an Atkinson engine), which extracts as much of the work as possible (similar to diesel expansion). This gives very good fuel economy at the loss of specific power (so the engine is a low power 2.0L, but the electric motor supports the acceleration, so whilst not a sports car, it is good) 

2: The hybrid gearbox operates as a CVT so you can run the engine at the most efficient point for any power level. 

3: At very low power levels when the conventional engines would be very inefficient the hybrid runs electric or worst case it runs efficently and charges the battery.

4: Regenerative braking. The better hybrids now run series brakes which means all but severe breaking is done by regeneration and energy recovery (also saves brake wear and reduces brake dust)

5: Also remember a litre of petrol has about 10% less energy content than a litre of diesel so a fair comparison is to expect 10% less mpg for a petrol if you want to talk efficiency. Sadly not reflected at the pumps so this point is often missed 

6: Further petrol has more hydrogen and less carbon than diesel so is slightly less poluting for global warming  (both are bad, but petrol slightly less bad)

As a slight note diesel gets better mpg than petrol due to expansion ratio, throttling loss (a diesel engine doesn't have to work hard against a throttle) and the fuel energy density.

The first two points get addressed by the full self charging hybrids and so the hybrid is the better option. I have owned both and will be staying hybrid whenever I can.

There are a significant number of ways in which a hybrid can achieve a big reduction in fuel consumption and perhaps more importantly CO2 Emissions. I can talk about the system that Ford and Toyota use (they use a common technology) I had both a Ford Kuga PHEV and a Kuga diesel. The PHEV was great, significantly better than the diesel even if I didn't charge it. Charging gave me a 30 mile electric range with a smallish battery. Not all hybrids run this way and so it does vary from manufacturer to manufacturer, although there are common themes. Biggest points are

1:  the Ford and Toyota Engine is modified to have a very long expansion stroke (an Atkinson engine), which extracts as much of the work as possible (similar to diesel expansion). This gives very good fuel economy at the loss of specific power (so the engine is a low power 2.0L, but the electric motor supports the acceleration, so whilst not a sports car, it is good) 

2: The hybrid gearbox operates as a CVT so you can run the engine at the most efficient point for any power level. 

3: At very low power levels when the conventional engines would be very inefficient the hybrid runs electric or worst case it runs efficently and charges the battery.

4: Regenerative braking. The better hybrids now run series brakes which means all but severe breaking is done by regeneration and energy recovery (also saves brake wear and reduces brake dust)

5: Also remember a litre of petrol has about 10% less energy content than a litre of diesel so a fair comparison is to expect 10% less mpg for a petrol if you want to talk efficiency. Sadly not reflected at the pumps so this point is often missed 

6: Further petrol has more hydrogen and less carbon than diesel so is slightly less poluting for global warming  (both are bad, but petrol slightly less bad)

As a slight note diesel gets better mpg than petrol due to expansion ratio, throttling loss (a diesel engine doesn't have to work hard against a throttle) and the fuel energy density.

The first two points get addressed by the full self charging hybrids and so the hybrid is the better option. I have owned both and will be staying hybrid whenever I can.