Psionic clairvoyance may be using the x-ray or gamma ray part of the ems - it may well use dark radiation ( https://en.wikipedia.org/wiki/Dark_radiation ) for all we know, or it may involve perception via the psionic dimensions.
X-ray or gamma ray would need an emiter to allow to see through them (Superman vision notwithstanding)...
Abaut psionic dimenisons, whatever they might be, as you hint we have no information, but nowhere (AFAIK) is explained (or refered) that clairvoyence might be used to see in darkness (e.g. to offset darkness modifiers), neither clairaudience to hear other soundwaves than usually allowed (though I must admit neither is said anything agains it, again AFAIK)...