Undefeated heavyweight contender Anthony Joshua has 15 knockouts in his 15 professional fights.  Check out this video compilation from SHOWTIME Sports showcasing all of Joshua’s KOs.  And tune in this Saturday to watch the 2012 British Olympic Gold Medalist challenge undefeated American titlist Charles Martin for the IBF Heavyweight World Championship, live on SHOWTIME at 5 p.m. ET/2 p.m. PT from The O2 in London.