Why is the lamb or kid used on Pesach called a Korban? None of its meat is burnt on the altar/mizbe’ach. In what way is it a ‘sacrifice’?
It is a sacrifice (korban) because some of the animal (a lamb or goat male in its first year) is offered on the altar. The Rambam writes (MT Korban Pesach 1:6)
The blood of the Paschal sacrifice should be poured out on the base of the altar. After the blood was poured, its belly should be opened up, the fats and organs to be offered on the altar removed.
see also 1:14.
For a very interesting description of the entire Korban Pesach process, see here from dinonline; the bottom section "Faster than a speeding bullet" has the information on the burning of some internal organs and fats.
PS. Maybe you meant to ask about the first Korban Pesach offered in Egypt since its organs couldn't be offered on a mizbeach that didn't exist then. The gemara in Pesachim 96a answers that the door lintels of the Jewish homes were actually the altar (see also here).
"The blood of the Paschal sacrifice should be poured out on the base of the altar. After the blood was poured, its belly should be opened up, the fats and organs to be offered on the altar removed. The fats of each Paschal sacrifice should be offered on the pyre individually. The owner of the sacrifice should take his Paschal sacrifice with its hide to his home in Jerusalem.There he roasts it and eats it in the evening."
- Rambam Hilchos Korban Pesach 1:6
"If one set aside a female animal for his Paschal sacrifice or a male animal in its second or third year of life, it should be allowed to pasture until it contracts a disqualifying blemish. Then it should be sold and a Paschal sacrifice brought with the proceeds of the sale. If it did not contract a blemish until after he offered his Paschal sacrifice, he should bring a peace-offering with the proceeds of the sale."
- Rambam Hilchos Korban Pesach 4:4
The Korban Pesach is learned from the Oral Law to belong to the Todah-Shelamim family of offerings (due to their similarities). As you can see from the second entry of Rambam above, the Passover is able to have its funds interchanged to a peace offering under certain circumstances.
For the procedures of the peace and thanks offerings see VaYikra 3 and 7: 11 - 18. The peace, thanks, and Passover offerings are all called "kodoshim kalim" (holy offerings of lesser sanctity) and share many aspects (and some differences of course).
I hope this helps. :-)