Korcula is too far for a day trip from Hvar unless you pay for an excursion company to take you. Not worth it imo, when there are so many other options.
What you could do is ferry from Hvar to Korcula (note the car ferries depart from Stari Grad on Hvar), check out the town, then, if without a car, probably stay one night, then take the bus to Dubrovnik in the morning. It would be worth it just for the pastries, but the town is particularly lovely at sunset (all that golden stone).
If you rent a car while in Dubrovnik, you can drive to Korcula (about 3 hours).
You will love Hvar, and since it is so conveniently located, you can easily explore other places if you get bored with relaxing on the island. Just play around with those ferry schedules until you get a good plan--though jadrolinija isn't the only ferry, their website is helpful, as it has the map of the ferry ports.
Some ferries/catamarans arrive at Vela Luka on Korcula--a bus then departs from there to Korcula town (about 45 minutes away).

Wow, Yorkshire,

I never thought of taking the bus from Korcula to Dubrovnik. That would solve all of the problems. I believe I made note that I can get from Hvar to Korcula daily, but the return was a problem.

If we took the bus onward, that alleviates alot of the schedule issues.

Thank you VERY much!
h

We were in Croatia in September and October. We took a ferry from Venice, rented a car in Rovinj, Istria, mostly to get around the Istrian peninsula and to get to Plitvice Lakes, drove to Split and turned the car in. From Split, because the ferry schedule is less frequent in late September, we took the bus to Dubrovnik, 4-5 hours, a beautiful trip down the coast. We returned to Split by bus, took the ferry to Hvar for a few days, then back to Split. We really did very well without the car for most of the trip. When in Dubrovnik, we walked from our apartment into the old town, and we took a one-day bus tour to Montenegro. We had planned to do that trip on our own on the bus, but it turned out that we would have taken the bus to Kotor, spent an hour there, then taken the bus back to Dub. That was the only bus available. The bus trip cost $88, and it included lunch in Budva, a most beautiful town. The driver and tour guide were wonderful!

Your itinerary sounds great. You'll have a wonderful time either way. We only traveled between Dubrovnik and Korcula, bu I really loved having a car when we were on Korcula. I felt like we sound so many coves and villages we would have never otherwise seen. We posted a trip report on www.hitherandthither.net. Hope you find some of it helpful!