In a medium saucepan, heat with with sugar over medium-high heat, stirring occasionally until sugar is dissolved. Simmer, uncovered, 5 minutes. Remove from heat and stir in mint leaves. Let stand 20 minutes.
Meanwhile, peel kiwifruit and cut into chunks. Puree in food processor. Place puree in a pitcher. Strain cooled syrup into pitcher, pressing on mint, then discard leaves. Refrigerate until cold. Squeeze juice from 2 lemons. Stir into kiwifruit mixture. Taste, squeeze in juice from remaining lemon for a tarter lemonade. Pour into glasses. Top with sparkling water. Serve garnished with a slice of kiwifruit. Makes 2 1/4 cups without sparkling water - 4 glasses.