There are two type of kanji compound in a word:
1. Kun+Kun or On+On
This compound called 湯桶読み（よとうよみ, yotouyomi）. According Hirata-sensei (my Japanese teacher), this type of compound is approximately 90% of Japanese word.
2. Kun+On or On+Kun
This compound called 重箱読み(じゅうばこよみ, jyuubakoyomi）. This type of compound is approximately 10% of Japanese word.
Kun : means Kun-yomi (Japanese reading)
On : means On-yomi (Chinese reading)