Tomlinson and Moon (2013) suggest:

Two common issues with assessment, grading, and differentiation fall into the misconception category. The first has to do with content goals or KUDs, the second with a "standard" for grading and reporting. 
Some educators think of differentiation as having different goals for different students. Not only does this perspective make teaching and learning much more confusing and complicated, but it turns grading into a nightmare. It's essential to know that defensible differentiation seeks to provide multiple pathways and support systems to the same content goals so that virtually all learners can achieve higher levels of success with the same essential knowledge, understanding, and skill. Thus a teacher in a differentiated classroom is not grading students on different goals (with the exception of students with certain IEPs), but rather will provide feedback and grades based on a student's status relative to the same KUDs.
Further, some educators feel as though differentiation calls on teachers to grade struggling students "easier" and advanced students "harder."  That, too, is a misconception.  Differentiation is not about juggling grades....virtually all students in a differentiated classroom should be graded against the same clearly delineated criteria (KUDs) (p. 126).

Need a visual of this model in practice?  Guskey and Jung have it covered in their 2010 Educational Leadership article entitled "Grading Exceptional Learners."  The visual below is referenced in the article.