The main difference is locality. H. fulvipes is from Pakistand, but I forget where H. xanthopus is from. Check George's "How To ID Common Heterometrus" thread. IT has the granulation pattern for fulvipes.
Here is a more detailed version, copied from the above link...
**************** Pictured on the left Heterometrus xanthopus (Pocock, 1897)
DIAGNOSIS. Adults 60–80 mm long. Color of adults
uniformly reddish brown, legs and telson yellow to
yellowish or reddish brown, always lighter colored than
body. Pectinal teeth number 13–16 in both sexes. Male
has longer femur and patella of pedipalp. Chela lobiform,
in male narrower, its length to width ratio 2.1 in males
and 1.6 in females. Entire manus covered by rounded
minute granules that may merge but are devoid of
carinae. Patella of pedipalp without pronouced internal
tubercle. Carapace in juveniles with disc smooth,
margins granulate and anterior portion tuberculate; in
adults entire carapace sparsely granulated. Vesicle of
telson as long as or longer than aculeus.
COMMENTS. Couzijn (1981: 136) stated that the type
series consists of the holotype, allotype and three
juveniles. However, he did not designate them, and they
had not been designated by the original describer
(Pocock, 1897: 116) either. Consequently, in order to
stabilize the nomenclature of the species, I hereby
designate one of the examined syntypes as the lectotype.
Pictured on the right Heterometrus fulvipes (C. L. Koch, 1837)
DIAGNOSIS. Adults 70–100 mm long. Color of adults
uniformly reddish brown to black, legs and telson yellow
to yellowish or reddish brown, always lighter colored
than body. Pectinal teeth number 12–18 in both sexes.
Male has longer femur and patella of pedipalp. Chela
lobiform, its ratio between 1.7 and 1.9 in both sexes.
Male and female chela of different shape but
approximately same width. Entire manus covered by
large, rounded granules but devoid of carinae. Patella of
pedipalp without pronouced internal tubercle. Carapace
of juveniles and some females with disc smooth, at
margins granulate and anteriorly tuberculate. Males and
some females with entire carapace sparsely granulate.
Telson hirsute, vesicle longer than or as long as aculeus.
DISTRIBUTION. India: Andhra Pradesh, Bihar, Gujarat,
Karnataka, Madhya Pradesh, Maharashtra, Orissa,
Rajasthan, and Tamil Nadu (Pocock, 1900: 89, Couzijn,
1981: 134, Tikader & Bastawade, 1983: 603); Mysore,
Uttar Pradesh, and West Bengal (first report). The type
locality originally given as Java (C. L. Koch, 1837: 46)
is considered erroneous (see Fet, 2000: 439). Also
erroneous are records from Sri Lanka (Pocock, 1900: 88)
and Myanmar (Kopstein, 1921: 133).