There are many mechanisms involved, but the main are increased stability of proteins and cell membranes. For instance, archaeal membranes consist of lipids containing ether bonds, whereas most of bacterial and eucaryotic membranes contain lipids with ester bonds. Lipid membranes with majority of ether-containing lipids can stay in liquid-crystal state at a wider range of temperatures, and in helps the archaea survive in such conditions. 

There are many mechanisms involved, but the main are increased stability of proteins and cell membranes. For instance, archaeal membranes consist of lipids containing ether bonds, whereas most of bacterial and eucaryotic membranes contain lipids with ester bonds. Lipid membranes with majority of ether-containing lipids can stay in liquid-crystal state at a wider range of temperatures, and in helps the archaea survive in such conditions. 

Most of time they have protection around them and thermostable enzyme to do the function.

Thermophilic bacteria have adaptation of all key mechanisms

1) DNA and mRNA adaptation (high GC, codone adaptation, different protein-binding properties) 

Nakashima H, Fukuchi S, Nishikawa K. Compositional changes in RNA, DNA and proteins for bacterial adaptation to higher and lower temperatures. J Biochem.

2003 Apr;133(4):507-13. 

Fujita M, Kanehisa M. Comparative analysis of DNA-binding proteins between

thermophilic and mesophilic bacteria. Genome Inform. 2005;16(1):174-81.

2) tRNA structure and protein synthesis mechanism adaptation

Dutta A, Chaudhuri K. Analysis of tRNA composition and folding in

psychrophilic, mesophilic and thermophilic genomes: indications for thermal

adaptation. FEMS Microbiol Lett. 2010 Apr;305(2):100-8. doi:

10.1111/j.1574-6968.2010.01922.x.

 Gruber T, Köhrer C, Lung B, Shcherbakov D, Piendl W. Affinity of ribosomal

protein S8 from mesophilic and (hyper)thermophilic archaea and bacteria for 16S

rRNA correlates with the growth temperatures of the organisms. FEBS Lett. 2003

Aug 14;549(1-3):123-8. 

3) enzymes

Fukuchi S, Nishikawa K. Protein surface amino acid compositions distinctively

differ between thermophilic and mesophilic bacteria. J Mol Biol. 2001 Jun

15;309(4):835-43. PubMed PMID: 11399062.

4) structural lipids and proteins

 Yosuke KogaThermal Adaptation of the Archaeal and Bacterial Lipid Membranes Archaea Volume 2012 (2012), http://dx.doi.org/10.1155/2012/789652

5) Other mechanisms (secretion systems, recombination, DNA uptake, secondary methabolism).

thank you for your answer