I would guess most icelandic manuscripts were produced using sheepskin, that
would be the most readily available. It is worth noting that sheep and
goatskin are often called parchment rather than vellum. This comes from the
name of the city where the slightly different curing process used was
invented. Sheep and goatskin can still be called vellum if they are produced
using the same process as 'real' vellum

As to the relative merits of each material:

The parchments were made using a cheaper process which yielded comparatively
poor results, some manuscripts using parchment will show signs of earlier
texts where the ink was scraped off and the parchment reused.

Sheepskin (and goatskin) vellum were cheaper than other vellums due to their
relatively high availability and could be of high quality, they tended to be
rougher on the skin side than calf vellum.

Calfskin was used primarily for very large books (many Graduals and Gospels
were made using calfskin for this reason) but was also used for some high
quality manuscripts. Calfskin was from a younger animal and as such tended to
be naturally higher quality, it was also marginally thicker than sheepskin
and could therefore be made smoother by scraping away more surface material.

Kitskin could mean several things but most often referred to either the skin
of mink, foxes, rabbits etc or of unborn animals. The skin from unborn
animals yielded the very highest quality vellum but was prohibitively
expensive, such vellum is almost pure white and, due to better treatment,
surprisingly hardwearing and supple.

I could go on to describe curing processes etc but somehow I doubt anyone
cares ;-)