Ms. Joshi has raised a good thread. Let me try to mention some basic facts very briefly:-
(a) No country (either India or Pakistan) would go for total war at the first instance.
(b) Both the countries will first engage in limited warfare in a limited area with limited objective (example: Kargil).
(c) If the objective fails, or if the limited war causes unacceptable loss for one country, it would go for total war (example:India in 1965 war).
(d) In case of total war, both parties would start with conventional warfare.
(e) If the conventional war is not finished within a reasonable time, Pakistan would face the threat of implosion from inside and explosion from outside. India's superior war machinery will start talking and the internal disintegration threat at Baluchistan and Sindh would become a reality. Then Pakistan woul resort to nuclear strike.
(f) To nuke India, Pakistan would generally rely upon its missiles like 'Shaheen' and 'Hatf', which are basically North-Korean/Chinese missiles with questionable accuracy. To increase the accuracy Pakistan would use these in Indian border states like Punjab, Rajasthan or Gujarat. It is unlikely for Pakistan to attempt to nuke Mumbai at the first instance. Pakistan would successfully use 2-3 nuclear weapons with low to medium level of destructive power (Kilotons). Only a fraction of India's territory and about 10 % of the total population would be affected by Pakistan's strike. Pakistan would not try to use its Air Force because its Air Force is technically incompetent to do so (carrying nuclear weapons on board) and the country is aware that India has a credible air defense system.
(g) To discuss India's retaliation, we must remember that India has 'no first-use' nuclear policy. So, India is strenthening its second-strike capability over last 20 years or so.
(h) India's response to Pakistan's strike would be swift, brutal and totally asymmetric. India would use nuclear submarines, medium range air to air, air to ground missiles and Air Force to nuke Pakistan.
(i) Before any international intervention, at lease 5-8 nuclear devices would be used by India. Most likely targets would be Karachi, Lahore, Sialkot, Multan and Peshawar.
(j) The destruction would be very lethal as more than 60% of Pakistan's entire population live within 100 kms. of Indian border.
Non-violence is the greatest Dharma; So too is all righteous violence.