That's a difficult question to answer without knowing what format XML you want. System Events has an XML Suite, but I'm not sure it will give you an XML string for anything but a complete document. It's also incredibly slow for anything not very simple.
And the other option is AppleScriptObjC. Here's something very simple:
use AppleScript version "2.4" -- Yosemite (10.10) or later
use framework "Foundation"
use scripting additions
set theList to {"a", "b", 1, 2, 3.3}
set listElement to current application's NSXMLNode's elementWithName:"array"
repeat with anItem in theList
set newNode to (current application's NSXMLNode's elementWithName:"entry" stringValue:anItem)
(listElement's addChild:newNode)
end repeat
return listElement's XMLString() as text