EDIT:
An example of a Stotra that is a Mantra but not a Kavacha, is the Devi Khadgamala Stotram.
It starts with the Viniyoga:
Om Asya Sri Suddha Sakti málá mahá mantrasya
Upasthendriya adhisthayi Varunaditya Rishih
Devi Gayatri Chandah
Satvika Kakára bhattaraka pithasthita
Sri mat Kámesveranka nilaya
Sri mat Kámesvari devatá
Aim Bijam, Klim Saktihi, Souh kilakam
Sri Devi prityarthe Khadga siddthyarthe jape viniyogah
To the pure garland of prayers to Goddess Shakthi ,
Which should be worshipped by all sense organs,
The Sage is the rising Sun,
The meter is Gayathri and the goddess presiding is Goddess Kameswari ,
Who exists in the peaceful Kameswara who is sitting on the seat of Kakara Bhattaraka,
The root is Im , the power is kleem , Sopu is the pivot ,
And is being chanted to please the Goddess and getting her sword.
It is of course a Stotra, as the name itself suggests, but at the same time it is a Mantra (a Mala Mantra to be precise since it has more than 20 syllables) as well.
But it is NOT a Kavacha since it does not have the aforementioned feature of a Kavacha.
So, in this way you have to differentiate between a Stotra and a Kavacha.