Ceramic tile is obviously a baked clay product. Porcelain is also a ceramic tile but of a slightly higher quality over standard ceramic in that the recipe differs and the product is fired at a higher temperature. Porcelain has the least absorbency of any tile but since most all ceramic floor tiles have a low absorbency this usually isn't a major benefit.
Porcelain tile is made to mimic most all other forms/designs of ceramic and stone tile.
Travertine on the other hand is a natural stone. Travertine is kin to marble and limestone and is at the opposite end of the absorbency spectrum from porcelain. Therefore in my opinion travertine wouldn't be the best choice for a kitchen floor.
Travertine tho it can be sealed would still be vulnerable to stains and acid etching over time, whereas porcelain would not.
It is also worth noting that travertine being a natural stone requires a subfloor with double the strength of that required for ceramic. Most late model homes aren't built to accept natural stone tiles without some subfloor modifications.
Typically travertine tile costs more than porcelain tile.
Travertine is beautiful but not always practical in a kitchen.
Porcelain is a product you can't go wrong with and it is available in a travertine look.
Hope this clears it up for you.