All the major cities of India like Mumbai, Delhi, Kolkata, Ahemdabad, Banglore are well connected by flights to Bangkok. You can then take a bus or taxi to reach Phetchaburi. Air Asia, Air India, Indigo, Jet Airways, Thai Airways are few of the popular airlines.
You can fly into Bangkok's Suvarnabhumi Airport from your respective city, and then take a bus or taxi to get to Phetchaburi.
A taxi ride from Bangkok to Phetchaburi takes the same amount of time as the bus, but is very expensive costing around 2,000 Baht, hence not very advisable.
There is a train from Hua Lamphong to Phetchaburi, but it takes very long and is not usually preferred.
A bus or a mini bus can be taken from Bangkok's Mo Chit bus station to Phetchaburi. The journey takes around 2 hours and costs 120 Baht.