There are numerous bus stations in Kiev and various companies offering services to and from international destinations.
Ecolines operates from the Central Bus Station on Moskovska Square (near Demiivska Metro Station) and offers buses to Kiev from European countries including Austria, Czech Republic, Denmark, Estonia, France, Germany, Hungary, Latvia, Netherlands, Poland, Slovakia, and the UK.
Passengers should note that northerly routes to Moscow and destinations in Latvia and Estonia (Riga, Tallinn etc.) pass through Belarus. Most travellers, with the exception of those holding a CIS passport, will need to obtain a transit visa to pass through Belarus. They are expensive and a pain to apply for.
Eurolines offers regular bus services from many European cities to Kiev and other destinations in Ukraine. Select your country of departure from the website to go to the country-specific site.