Hymns are traditional poems, many are taken from the Book of Psalms. They have been around for 100s of years, and are sung by congregations, while worshipping God in public. Carols, on the other hand, are festive songs. 

What makes a song a carol?

A carol is a festive song, generally religious, but not necessarily connected with church worship. The verb caroling (or "to carol") also refers to the singing of carols.

What is a hymn in music?

A hymn is a type of song, usually religious, specifically written for the purpose of adoration or prayer. The word, hymn derives from Greek ὕμνος (hymnos), which means "a song of praise".

Is a Christmas carol a hymn?

A Christmas carol (also called a noël, from the French word meaning "Christmas") is a carol (song or hymn) whose lyrics are on the theme of Christmas.