What's The Difference Between 'Even If' And 'Even Though'?

In popular informal usage there isn't a difference worth worrying about.  But if you want to speak elegantly or precisely, you might consider the differences that are implied by the different choices of wording.

If: implies that the action or fact following in the next clause is not certain.  Suggests more uncertainty about the preceding clause, too.

Though: implies that the following action/fact doesn't change the preceding clause; in fact, emphasises the certainty of the first clause.

Examples:

I may go swimming, even if it makes my mum nervous.

Suggests that the idea of swimming is not definite, as well as the fact that mum may or may not be nervous about it.

You could also say

I AM swimming later, even if it makes my mum nervous.

Which still sounds like it isn't certain that you know it will make mum nervous.  That also puts a slight question mark over the resolve to actually go swimming.

I am swimming later, even though it makes my mum nervous.

Makes it clear that speaker knows mum will be nervous for certain, and is acting in spite of that certitude.

"Even if'' refers to a possible outcome, whereas 'even though' refers to a definite outcome. The sentence 'I am going for a picnic even if it is raining' is different to 'I am going for a picnic even though it is raining.' In the first sentence, it may or may not rain. In the second sentence, it is raining. There can also be a subtle difference in tense when using the phrases 'even if' or 'even though'.  The latter infers that you have some experience of the outcome already. It is either happening in the past or is currently happening. 'Even if,' on the other hand, usually hints at an outcome in the future. You don't know what that outcome will be; therefore the scenario you are referring to may or may not happen. Both 'even if' and 'even though' can have a hint of defiance.  The statement, 'I am going on a picnic even though it is raining,' shows that you are taking an action despite a certain outcome.