We got to Znojmo by train from Vienna. We paid 16 Euros per person for a return ticket (you must return within four days on this ticket). We boarded the train at Wien Mitte Station, but the train started from Wien Meidling Station. The journey took 1 hour 33 minutes from Wien Mitte Staiton. It was comfortable and the train was fairly empty and very clean.
Every second hour a train from Vienna, 1.30 hours € 16 with EURegio ticket (return, 4 day´s valid, available at ticket counters and vending machines), € 4 from Retz.
There are frequent trains from Breclav and Okrísky but no direct trains from Brno or Praha.