Read here to understand what a Harthal is.
This must be a common question for many of the tourists visiting Kerala. There is no one word answer for this. The issues you might face depends on the situation on which the Harthal is being declared and the place where the trigger for the harthal had evolved. There are mainly 2 types of Harthals, first being called for the complete state and other being for a District or a Locality or City. In the State Harthals it will be very difficult for you to enter or travel through the State.
Harthals are usually declared just one or 2 days prior, which will make it very difficult for you to arrange alternatives. So do either of these, 1) Make sure you reach the place the prior day night of Harthal and check in or 2) push your arrival after 6:00 PM on Harthal day.
Most of the cases you will witness a total stand still, which will be a Fun Killer for you. If you are commuting you will surely face trouble finding Restaurants or shopping of any kind on the route. Only facilities working that day will be the Police and Hospitals. You will be better of not on the roads that time period. Just stay wherever you were staying the previous day night.
This will also depend on the type of vacation you are into. If you are spending time in a resort or a package tour where there will be zero commute involved, then its most likely that you are not impacted.