In a blender, puree the diced watermelon with the fresh lime juice, chervil leaves and extra-virgin olive oil until the mixture is very smooth. Set a fine-mesh strainer over a large bowl and strain the watermelon puree, pressing very lightly on the solids to extract the watermelon juice without pushing the pulp through.
Rinse out the blender and return the strained watermelon juice to it. With the blender on, add the xanthan gum and blend until slightly thickened, about 15 seconds. Transfer the watermelon soup to a pitcher and season with Tabasco and salt. Refrigerate until the soup is well-chilled, about 20 minutes. Serve the soup in bowls, garnished with chervil or shredded shiso leaves.