One of the fastest way of getting XP in farmville is by buying buildings and spending the coins.
But while buying building we spend 100 coins for 1 XP. I agree the bonus is that we get a building. But we don't have so much space in our farms
to keep so many buildings so ultimately we store it in the store house.
But the cheapest and fastest way of getting XP is BUYING HAY BALES.
I wouldn't have suggested this to any one earlier but ever since the delete continuously option has been introduced in farmville it has been much easier to place hay bales in the farm and than delete it continuously.
There are lot of types of hay bales available in the market.
But I would suggest that if you want to place hay bale only for XP than you should place simple hay bale that cost you 125 coins and gives you 5 XP. That is like 1 Xp for 25 coins, which in comparison with 100 coins for 1 XP is 4 times cheaper.
It may not be very easy but is definitely effective.