Caveat: I know nothing about cvtest or cvsim. I suspect that cvsim is trying to inspect the calling workspace - this is not allowed inside parfor, and causes a "transparency violation". (This slightly cryptic term simply means that all accesses to variables must be "transparent" - i.e. obvious in the text of the program.)
Often, the simplest route is to pop the body of your parfor loop into a separate function, so you'd do:
parfor idx = ...
out(idx) = myCvSim(idx,model)
end
...
function out = myCvSim(idx,model)
testObj = cvtest(model);
...
out = cvsim(testObj);
end
