Haar measure is a measure on locally compact abelian groups which is invariants to translations. For example, the Lebesgue measure on the reals is such measure.

It can be shown without the use of the axiom of choice that the Haar measure exists and it is unique up to a scalar, that is if we want the measure of the unit interval (for example) to be $1$ then it is really unique.

While the measure is defined on Borel subsets, but we can complete the measure in a unique way by adding all the subsets of measure zero sets (and in the case of the real numbers we once again have the Lebesgue algebra)

As with the Lebesgue measure, when the axiom of choice is present there are cases in which non-measurable sets can be constructed. In the Solovay model, however, we have that all subsets of reals are measurables.

Are there any similar results about Haar measures of general LCA groups? Is there a model in which all Haar measures (perhaps under some limitations on the groups) are "full measures" (in the sense that every subset is measurable)?

abeliangroups? If I'm not completely mistaken Cartan's existence proof of Haar measure, as presented e.g. in Hewitt-Ross,Abstract Harmonic Analysis I, Theorem 15.5, p.185ff, is entirely constructive and it applies toanylocally compact (Hausdorff) group. $\endgroup$ – Theo Buehler Jul 17 '11 at 13:00