The currency of Kerala is Indian Rupees.
Similar to 100 Cents making up 1 US Dollar, 100 Paise makes up 1 Rupee (Rs).
Rs 1, 2 and 5 comes in coins. Rs 10, 20, 50, 100, 500 and 1000 are the currency note denominations in circulation.
Credit/Debit cards are not accepted everywhere. So it is absolutely necessary to carry some cash. And have to be mindful of pick pockets too! Keep some loose change as you may need it for public transport.
Below is a currency converter from Currenciesdirect. Rates you get within Kerala may vary.
In addition to banks, there are money exchanges in most towns. UAE exchange is the prominent one, with a wide network of branches. Usually I use them for exchanging money while in Kerala.
Western Union has a good market presence. Western Union has teamed up with the Post Office, so that you can receive money transfer from the Post Office.
You will find ATMs in most towns. However, tourist locations such as waterfalls, wildlife sanctuaries, remote beaches etc do not have any ATMS.