From HoNWiki

[edit] Description

Experience is gained from killing enemies, or by being close enough to them when they are killed.

It is possible to Deny enemy heroes some of the experience from a kill by last hitting an allied unit (does not work on heroes). If you successfully Deny a friendly creep "Denied!" appears above the creep's corpse and enemy heroes in range only get partial experience from it. Melee heroes only get 50% of the experience, ranged heroes only get 25% of the experience. Denying also ensures that enemy heroes do not collect the gold bounty for killing the creep.

[edit] Experience Table

This table shows experience required per level, and experience given by killing an enemy hero of that level. As of Patch 0.1.34, if you are within 1000 units of an enemy hero's death, you also gain partial experience for the kill.

Hero Level	Total Experience Required	Hero Kill Experience	Hero Death Experience (within 1000 range)
1	0	100	12
2	200	120	24
3	500	160	36
4	900	220	48
5	1400	300	60
6	2000	400	72
7	2700	500	84
8	3500	600	96
9	4400	700	108
10	5400	800	120
11	6500	900	132
12	7700	1000	144
13	9000	1100	156
14	10400	1200	168
15	11900	1300	180
16	13500	1400	192
17	15200	1500	204
18	17000	1600	216
19	18900	1700	228
20	20900	1800	240
21	23000	1900	252
22	25200	2000	264
23	27500	2100	276
24	29900	2200	288
25	32400	2300	300