The current Wikipedia entry for "Tautology (Language)" did not have a reference to circular reasoning. To get an overview, consider also entry for "Circular reasoning" and "Tautology (logic)".
Wikipedia currently says this about "tautology":
In literary criticism and rhetoric, a tautology is a statement which repeats the same idea, using near-synonymous morphemes, words, or phrases, that is, "saying the same thing twice".
Saying the same thing in multiple ways may help the listener understand rather than present an argument justifying what one has said.
Wikipedia currently says this about "circular reasoning":
Circular reasoning...is a logical fallacy in which the reasoner begins with what they are trying to end with. The components of a circular argument are often logically valid because if the premises are true, the conclusion must be true. Circular reasoning is not a formal logical fallacy but a pragmatic defect in an argument whereby the premises are just as much in need of proof or evidence as the conclusion, and as a consequence the argument fails to persuade.
If one is trying to convince someone rather than help someone understand the reasoning used in the argument could be circular. It should not persuade as an argument, but the hearer may come to a better understanding of the premises.
Todd's answer provides a good description of a logical tautology:
A tautology is a single proposition, not an argument, that is true due to its form alone (therefore true in any model).
Wikipedia has this to say about a logical tautology:
In logic, a tautology (from the Greek word ταυτολογία) is a formula or assertion that is true in every possible interpretation.
Todd's "single proposition" would be one way to paraphrase "formula or assertion".
Here is the OP's question:
Can someone elaborate how they are different? I used to think that circular reasoning was a subset of tautology.
Given these sources, circular reasoning is an argument that should not be persuasive if one is not already convinced of the premises. A rhetorical tautology would be a repetition or a way to better explain something by repeating it with possibly slight modifications. A logic tautology is a formula (well-formed formula, sentence, single proposition), not an argument, that can be assigned a true-false semantics so that a complete truth table of these valuations or interpretations would show that the formula is always true.
Todd's answer: https://philosophy.stackexchange.com/a/40857/29944
Wikipedia, "Circular reasoning" https://en.wikipedia.org/wiki/Circular_reasoning
Wikipedia, "Tautology (language)" https://en.wikipedia.org/wiki/Tautology_(language)
Wikipedia, "Tautology (logic)" https://en.wikipedia.org/wiki/Tautology_(logic)