Kythnos and Paros belong to different worlds within the Cyclades. Kythnos is on the western line from Piraeus and Lavrio, a quiet, very Greek island known for its hot springs at Loutra, its sandy bays and inland villages like Messaria (Chora) and Dryopida. Paros is in the centre of the group: a busy hub with Parikia as its port town, Naoussa's fishing harbour, and boats to almost every island in the Aegean.
Because they sit on different ferry lines, a direct boat between them is the exception rather than the rule. When we researched this page, Ferryhopper had no route page for Kythnos to Paros in either direction, and our dated timetable sample showed no direct sailing. Our older route data lists Seajets on the link as a seasonal service, but we could not match it to any dated departure, so we do not publish its crossing time, sailing days or fare.
That does not make the trip hard. The reliable plan is to go back to Piraeus and out again: Seajets reaches Piraeus from Kythnos in about 1 hr 30 min, and from Piraeus there are ferries to Paros every day of the year, the fastest taking about 2 hr 40 min. It can be done in a day when the connections line up, and Piraeus has more choice than any island in between.
Before you book anything, search your exact date in the booking box. If a direct Kythnos to Paros sailing appears, it will be the simplest option; if not, the booking search will show you the combinations through Piraeus or other islands. Leave a comfortable gap between the two boats, as the first leg can run late.