There are three big challenges in establishing what the relationship was between the Huns and the Xiongnu. The first challenge is the relative lack of information we have on both of them- there is no consensus on what language(s) they spoke, for example. The second challenge is the lack of written Central Asian sources which could tell us what the Northern Xiongnu did after they migrated West or what the Huns did before they arrived on the Pontic Steppe. The third challenge has to do with the nature of steppe powers in general- they are confederations often made up of many different tribal or ethnic groups, and one khanate does not necessarily equate to one language, one culture or one ethnic identity. With such large gaps in the available evidence, we can only make educated guesses about their relationship.

The Huns may have been a new confederation which included descendants of the Northern Xiongnu. They may also have been a continuation of the Northern Xiongnu state (the scholar Hyun Jin Kim argues for this, based on similarities he sees between Hunnic and Xiongnu political structure), but in that case the 'ethnic Xiongnu' could have possibly only made up a small ruling group rather than the whole body of Hunnic people. Or, the similarities between the names used by various peoples to refer to the two could be the result of settled peoples misunderstanding or even stereotyping steppe peoples, and not an indicator of a genuine close connection between the two. There's just not enough evidence to say firmly which of these is true.