It is very important to distinguish between the concepts of "native language" and "language, one prefers to speak in everyday life". The other answer to the stated question, used native language percentage as the source of their claims - that's deceiving, since many people who consider a language their monther tongue don't necessarily use it in everyday life (both at work and outside of work). Therefore, the answer I'm providing is based on socialogical survey of Kyiv residents that provides data on the "language, one prefers to speak in everyday life".
Considering the data I've seen (both academic papers based on sociological surveys and my personal obervation of Ukrainians in everyday life), I would dissuade you from going to Kyiv to learn Ukrainian and would encourage to go to Lviv instead. Lviv has roughly ~90% Ukrainian speakers (a 2001 paper published in an an influential "Ji" magazine using a sociological survey of Lviv residents in 2000 stated that 89% of Lviv city dwellers preffered to use Ukrainian in everyday life; source in Ukrainian) and you would hear Ukrainian spoken everywhere - thus giving you plenty of opportunities to pick up Ukrainian. Kyiv, on the contrary, is highly russified (a 2013 paper from the The National Academy of Sciences of Ukraine stated that among Kyivan dwellers, 32,43% percent use Russian as a language they are compfortable to use in everyday life while 17,57% percent use Ukrainian as a language they are compfortable to use in everyday life (with the rest being indifferent between the two, which knowing Ukrainian realities means they are most likely to prefer Russian language in everyday life); source in Ukrainian). Thus you will have considerably less opportunities to learn Ukrainian in Kyiv (since fewer Kyiv dwellers prefer to use Ukrainian than Russian in everyday life) compared to Lviv.
Lastly, as Yuri Shevchuk (associate professor at Columbia University who routinely teaches Ukrainian language courses there) pointed out in one of his interviews, when some of his Ukrainian courses' students travel to Ukraine looking to practice their spoken Ukrainian, he warns them that they will find it hard to practice Ukrainian in Kyiv, Kharkiv or Odesa (source in Ukrainian).