Kill rewards is an easy to use plugin that allows server admins to execute certain actions when a player reaches a certain killstreak or number of kills.
You can download the latest version of Kill Rewards here.
// Obtaining an instance
KillRewards killRewards = (KillRewards) Bukkit.getPluginManager().getPlugin("KillRewards");
// Getting the number of kills a player has
Player player = ...;
int kills = killRewards.getKillsManager().getKills(player);
// Getting the kill streak of a player
Player player = ...;
int kills = killRewards.getKillsManager().getKillStreak(player);