Throw PlayerLevelChangeEvent and PlayerExpChangeEvent events. Thanks to feildmaster for the PR.

src/main/java/net/minecraft/server/EntityExperienceOrb.java

@@ -1,5 +1,7 @@
 package net.minecraft.server;
 
+import org.bukkit.craftbukkit.event.CraftEventFactory; // CraftBukkit
+
 public class EntityExperienceOrb extends Entity {
 
     public int a;
@@ -130,7 +132,7 @@ public class EntityExperienceOrb extends Entity {
                 entityhuman.x = 2;
                 this.world.makeSound(this, "random.orb", 0.1F, 0.5F * ((this.random.nextFloat() - this.random.nextFloat()) * 0.7F + 1.8F));
                 entityhuman.receive(this, 1);
-                entityhuman.giveExp(this.value);
+                entityhuman.giveExp(CraftEventFactory.callPlayerExpChangeEvent(entityhuman, this.value).getAmount()); // CraftBukkit - this.value to event.getAmount()
                 this.die();
             }
         }

src/main/java/net/minecraft/server/EntityHuman.java

@@ -57,6 +57,7 @@ public abstract class EntityHuman extends EntityLiving {
     protected boolean J = false;
     public float K;
     public PlayerAbilities abilities = new PlayerAbilities();
+    public int oldLevel = -1; // CraftBukkit
     public int expLevel;
     public int expTotal;
     public float exp;

src/main/java/net/minecraft/server/EntityPlayer.java

@@ -319,6 +319,17 @@ public class EntityPlayer extends EntityHuman implements ICrafting {
             this.lastSentExp = this.expTotal;
             this.netServerHandler.sendPacket(new Packet43SetExperience(this.exp, this.expTotal, this.expLevel));
         }
+
+        // CraftBukkit start
+        if (this.oldLevel == -1) {
+            this.oldLevel = this.expLevel;
+        }
+
+        if (this.oldLevel != this.expLevel) {
+            CraftEventFactory.callPlayerLevelChangeEvent(this.world.getServer().getPlayer((EntityPlayer) this), this.oldLevel, this.expLevel);
+            this.oldLevel = this.expLevel;
+        }
+        // CraftBukkit end
     }
 
     public void e(int i) {

src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java

@@ -391,4 +391,17 @@ public class CraftEventFactory {
 
         return event;
     }
+
+    public static PlayerLevelChangeEvent callPlayerLevelChangeEvent(Player player, int oldLevel, int newLevel) {
+        PlayerLevelChangeEvent event = new PlayerLevelChangeEvent(player, oldLevel, newLevel);
+        Bukkit.getPluginManager().callEvent(event);
+        return event;
+    }
+
+    public static PlayerExpChangeEvent callPlayerExpChangeEvent(EntityHuman entity, int expAmount) {
+        Player player = (Player) entity.getBukkitEntity();
+        PlayerExpChangeEvent event = new PlayerExpChangeEvent(player, expAmount);
+        Bukkit.getPluginManager().callEvent(event);
+        return event;
+    }
 }