I am trying to get an output varying from 0-1000V at load capacitor . For that , a PWM signal is applied to the gate of the MOSFETs . As the PWM signal is generated from the 555 timer , so the pulse signal voltage varies from 0-15V. But couldn't able to observe any switching operation. what minimum voltage need to be applied for the switching operation ? How can I make output to switch between 0 to 1000V.

 

Consider the gate-source voltage of Q4.
If you actually built this circuit it would destroy Q4.

 

The complimentary inverter is a poor choice for high voltage because of the overlap of the two transistors during switching transitions. Once you solve that you need a resistor between the switches and the out capacitor like in a buck converter (which is what you appear to be trying to make).

If you use an inductor between the switch and the output capacitor you can replace one of the switching transistors with a diode, thus eliminating the conduction crossover while switching.

Check out some buck converter circuits on the internet to see what I mean.

 

There are Half Bridge Gate Driver ICs that do this job. Then you use two MOSFETs of the same type.
This IC has a input for each transistor. There are some with one input. It will automatically turn off before turn on to keep both transistor from being on at the same time.
The down side is you can not leave the top transistor no 100% of the time. You can have the bottom one on all the time.
If you need help choosing a part just ask but please try first.

 

What amount of current is required, or is this a bit of homework, or possibly a test. Creating a switcher voltage regulator will not work with the circuit shown, for all of the reasons stated so far.