To reach Xanthi, one should fly to Kavala Airport (KVA). This is a local airport with connections to Athens. From the airport, taxi is the only way to reach Xanthi and costs about ~55€ including all extras.

Alternatively, you can fly to Thessaloniki Airport (SKG). From there, take the local (blue) bus X1 AERODROMIO–KTEL to the main bus station (approx. 1 hour, cost 0.90€). Then, take a (green) KTEL bus to Xanthi — they run about every 2 hours, cost around 19€, cover a distance of 214 km, and take about 3 hours.