According to SAOB, the official historic dictionary of Swedish, godkänd and försvarlig are much of an age; SAOB takes account only of what is printed so I think the cut-off is somewhere around 1530.
The meaning of godkänd and försvarlig is just as Heikki says; godkänd is pass while försvarlig, passable, doesn't quite cut it but is at the same time not a fail.
Put into context försvarlig seems to be adequate: Ingen, som icke inför sin pastor kunnat visa sig äga en försvarlig kunskap i katekesen, skulle få gifta sig. - Nobody, who before his vicar cannot be shown to have försvarlig/passable knowledge of the catechism, should be allowed to marry. (SAOB)
In grammar school in the 19th century the following marks were awarded:
Berömlig, Med beröm godkänd, Godkänd, Försvarlig, Otillräcklig/Ringa - Distinguished, With distinction, Pass, Passable, Inadequate/Poor. (SAOB)
Looking at this I would suggest that the terminology for the marks awarded during my schooldays (numeric, just to be different) was more realistic: 5 = Distinguished, 4 = With distinction, 3 = Average, 2 = Passable (which meant pass), 1 = Inadequate (this was in the 1970's, you were not allowed to say failed but that was of course what it was).
So I'd say that försvarlig, passable, actually meant pass while godkänd, pass, really meant average - if by pass you mean that you do not have to repeat the course/test and that it is enough not to limit or restrict you.
Ingela