What is the difference between partly and partially?

Generally, the words may be used interchangeably to refer to some amount or degree that is less than the whole. For example, you can say: She is partly responsible for their fighting. / She is partially responsible for their fighting. However, partially may imply favoritism because it is related to ‘partial’ and does have the archaic sense ‘in a biased manner’. To avoid ambiguity when describing favoritism, say: He is ‘partial’ to pizza. (Do not say: He was partially/partly favorable to pizza.)