In every playbooks in the Stat section they mention your maximum HP being: X + Constitution. Is it Constitution modifier or score?
It seems ambiguous to me.
When the text spells out an ability score, it means the score. When the text refers to the three-letter abbreviation, it means the modifier.
HP uses the Constitution score plus a class-specific value.
Dungeon World, page 17
Ability Scores and Modifiers
Many of the rules discussed in this chapter rely on a player character’s abilities and their modifiers. The abilities are Strength, Constitution, Dexterity, Intelligence, Wisdom, and Charisma. They measure a player character’s raw ability in each of those areas on a scale from 3 to 18, where 18 is the peak of mortal ability.
Each ability has a modifier associated with it that is used when rolling with that ability. These are written as three-letter abbreviations: STR, CON, DEX, INT, WIS, CHA. Modifiers range from -3 to +3. The modifier is always derived from the current ability score.
Usually, if they just mention Constitution, they mean the score. If they talk about the modifier, they explicitly tell you.
I found a link in another forum that mentions this about Dungeon World as well: http://rpggeek.com/thread/843474/ooc-dungeon-world-preventing-tentacle-embrace-is
Use the search function of your browser to find the comment about starting hp.