The /v/ in 'love' is a labio-dental fricative. That means that your top teeth are touching your bottom lip (think of biting your bottom lip) to constrict the air flow.
The /ð/ in 'loathe' is a lingua-dental fricative. This means that your tongue is touching the bottom ridge of your top teeth (think of licking the bottom edge of your front two teeth) to constrict the air flow.
In /v/ your mouth is mostly closed, because those top teeth have to bite down on that bottom lip. The lips themselves don't quite touch, because that bottom lip is tucked up behind the top teeth.
With /ð/, your moth is mostly open, although if you looked in a mirror, you'd see your tongue sticking out.
If that isn't clear, check out this site, and click on the /v/ and /ð/icons under the 'Fricative' tab. You can also click on the two blue arrows. The one in the middle of the page will show an animated cutaway view of a mouth as it makes the sounds, and the one on the right will show a closeup frontal view of the mouth as a person produces the sound.
Edit: Also, the /v/ sound should be easy enough for you, it's the same as the /v/ sound you already know in French. The /ð/ sound will be a new one for you since French doesn't have any labio-dental fricatives. There are actually two different kinds of 'th' in English - the /ð/ we've discussed above, and /θ̼/. The only difference between them is that /ð/is voiced (meaning there's a vibration in the vocal cords), and /θ̼ is voiceless (meaning the vocal cords don't vibrate). Neither exist in French, so they are new sounds you'll have to learn for English. They are formed the same way in regards to positioning of the tongue and teeth.
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